Toxic aquarium water & fish poisoning

Yellow fish

Signs of Poison

Toxicity levels vary depending on the species, the size, and the metabolic rate. The fish’s metabolism is more active in higher temperatures then in cooler environments, which allows toxins to act faster.

Toxic substances come in contact with the fish through the gills, rather than the skin. This allows most toxins to act very rapid as they enter the bloodstream of the fish very directly. Any degree of poisoning will weaken the fish, making it vulnerable toward disease.

Ammonia Poisoning

Ammonia is highly toxic to any fish. Reasons for an ammonia poisoning include a new set-up (nitrogen cycle), an interruption of the beneficial bacteria (i.e. power outage, medication, filter exchange) or a change in the bio-load, if too many new fish have been added to the aquarium too quickly.

The signs are a lethargic motionless fish hovering at the bottom of the tank, red gills and a lack of appetite. Advanced cases will show bleeding gills as well as external and internal bleeding toward the final stage before resulting in death.

In addition to a water change, first aid can be given by lowering the pH to 7.0 or less. Toxic ammonia changes into ammonium at this level and is, at this stage, harmless and non-toxic.

PH can be lowered by using distilled water for the partial water change.

Nitrite Poisoning

Nitrite is less toxic then ammonia, but still poses a significant health risk to the fish, and can be deadly in high doses or over long periods of time.

Fish gasping for air at the water surface could be a sign of nitrite poisoning.

Nitrite enters the bloodstream of the fish and binds hemoglobin cells – the oxygen carrying vessels of the fish’s body. In other words, high nitrite levels will suffocate the fish.

The cause for nitrite poisoning is the same as described with ammonia poisoning.

1 teaspoon of salt per 300 Gallons of water can help the fish to cope with nitrite toxins.

CO2 Poisoning

CO2 levels in excess of 25-30 ppm are dangerous for fish. Common signs for CO2 poisoning are an increasingand more rapid breathing, gasping for air, and a staggering swimming behavior – all leading to suffocation of the fish.

CO2 poisoning can be caused by a malfunction of the CO2 reactor, or the inability of plants to absorb CO2 if the lighting is insufficient.

A quick and long lasting solution is to heavily aerate the tank through surface agitation and air-stones. This will cause the CO2 to dissipate from the water.

Hydrogen Sulfite

In rare occasions, such as a severe lack of maintenance or an extended power outage, hydrogen sulfite can be formed in the gravel or within the filter.

Hydrogen sulfite can be detected by its rotten egg like smell. Hydrogen sulfite transforms iron within the blood cells into sulfide which will lead to suffocation of the fish.

The warning signs of hydrogen sulfite are the same as with nitrite poisoning in addition to the rotten egg smell of the water.

Prevention is crucial; the gravel should be cleaned on a regular basis. In case of a power outage, the filter has to be rinsed out well, before re-starting it. This will help eliminate the toxins that are in the filter, instead of washing them into the tank.

Chlorine, Chloramine, Heavy Metals

All three of them are in general of no concern as the water usually is treated with a conditioner prior to use, eliminating this problem right from the start.

Most tap water (city water) is treated with chlorine/chloramine to make it safe for human consumption. Unfortunately, our wet pets do not appreciate that. Chlorine/chloramine poisoning has similar signs as associated with nitrite poisoning. Chlorine/chloramine irritates the gills and blocks the oxygen carrying cells, again leading to suffocation. Additional to the fish gasping for air, a chlorine odor can be detected.

If so, adding a water conditioner is imminent, as chlorine/chloramine can kill all fish within 24 hours.

High concentrations of heavy metals can lead to a sudden fish death without any warning signs. A good water conditioner will also remove heavy metals next to chlorine/chloramine.

The degree of heavy metal toxicity is dependent on the water hardness. Fish can tolerate 10 times the amount of heavy metals in an aquarium at 18 degrees hardness as with 1 degree.

Some medications and aquarium additives contain copper; they have to be used with caution and should not be used over an extended period of time.

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46 thoughts on “Toxic aquarium water & fish poisoning

  1. Olivia says:

    My male beta fish died the day after 1 orbeez was accidentally dropped in the tank and I don’t know if it was because of the orbeez. I had immediately removed the orbeez. The fish was perfectly fine the day before and tank water was perfect. The next day the fish died. Could this be the cause!!!?????

    Please help!!!

    • Thilo
      Thilo says:

      Yes it could be the cause. Orbeez expand up to 300% and if the fish swallowed one, it could cause serious internal injury.

  2. Jill says:

    I got the fish a few months ago, and he was a baby, I clean my fish’s tank and all accessories including gravel daily… I feed my fish correctly… And he used to be really active, swimming around often and healthy… But all a sudden he gets tired often and lays at the bottom… But he has no symptoms of these besides laying at the bottom, there’s no bad smell or anything but he has been lazy and idk what it is!!!

  3. Katelin says:

    Hi my betta has started flashing and is pale. All the parameters were ok. Only real concerning was my ph jumped from low, to 6.2 over a single water change, and ammonia was .25. Could this rise in ph cause him to pale and flash??

    • Thilo
      Thilo says:

      Yes, and your pH is still very low, you might consider increasing it slowly by adding crushed corals to your substrate. The ideal level is 7.0 but anywhere close to that mark is acceptable. Fish tolerate a slightly next to optimum levels, but they will not tolerate fluctuations. It is always a good idea to know what the pH of the replacement water is. You might have to switch water sources if the pH comes in too low.
      If your pH jumped to 6.2 the beneficial bacteria might have been dormant or not fully operational, which would explain the ammonia. Ammonia is toxic and will affect the fish. If the ammonia persists for more than 2 days, you should add an ammonia detoxifier.

  4. Keauna says:

    Hey , so i have a heater for my tank and i forgot that i plugged It up and It caught my net on fire and so after that i stuck the heater inside of the fish tank , days later 12 of my fish died .. how would i clean this contaminated fish tank ?

    • Scott @ Algone
      Scott @ Algone says:

      I simply would start all over. Drain the tank and clean it out. Scrub all decorations and thoroughly rinse the gravel with lots of fresh water. If you use any cleaning products or bleach to clean the tank or decorations, make sure you rinse ALL residues before setting the tank up again.

  5. Auri says:

    Hi. My friend has a tank that was doing great. All of a sudden fish started dying and he thought it may have been because of a bad heater. He bought a new heater but fish are still dying and the water now has a bleach smell to it. Any idea what this could be? Nobody that’s around the tanks would ever think about bleaching the fish water. We’re all stumped.

    • Scott @ Algone
      Scott @ Algone says:

      It’s difficult to assess a cause with limited information. Your friends best bet is to test the water for ammonia and nitrite. Both are quite toxic and will lead to fish death if exposure is prolonged. One indication of either of these nutrients is fish gasping for air at the surface of the tank. Ammonia and nitrite inflame the gills, making it difficult for the fish to breath.

      If either of these 2 nutrients are present, he likely has a problem with the nitrogen cycle. It would indicate that nitrifying bacteria have died off in the tank, disturbing the biological process which breaks down accumulating organic waste. Bleach would certainly kill this needed bacteria. Consider the possibility that cleaning products have accidentally entered the tank while cleaning the outside of the aquarium glass or other cleaning activities around the tank.

  6. Oskar says:

    I could not find an article about this but it may be a problem from here. So about 2 weeks ago 1 of my Candal Tetra fish dissapred and my Platy looked fat. I assumed it was pregnant so i left it alone. When i went to Pets At Home to buy algae eaters, I asked the person how it dissapred she said that fish are cannibals. So i thought my fish died normaly. So about a cople of days ago another fish died 24 hours later 4 died and 24 hours after that I had no Canadal Tetras and 2 platy fish. Today 1 platy died and only the fat one left! That means it ate the other fish. Could anyone tell me what caused this aggressive behaviour in my platy??!!

    • Thilo
      Thilo says:

      If fish get weak they can fall prey to other, larger fish. I think it is not quite in the characteristics of platies to be predators. Some environmental adjustments maybe, make sure they have enough space and hiding spots. More females than males.
      Now, whenever fish die, water needs to be tested. Ammonia/ nitrite/ nitrates are of the utmost importance. Ammonia might actually be the culprit in the fish fatalities.

  7. Emily says:

    I’m looking for a poison that can kill fish and make the water cloudy because teacher bought 4 fish and put them in an empty aquarium and just the next day the water became cloudy and all the fish except one were dead and I wanted to help her by finding out what could’ve caused it but I couldn’t find anything I’ll ask her how the water smelled but that’s all I know right now

    • Scott @ Algone
      Scott @ Algone says:

      There are many things that could be responsible for the fish loss in this case. You may want to have a closer look at the aquarium nitrogen cycle. Every new aquarium needs to establish the nitrogen cycle when first started and during this time ammonia and nitrite will be present in the water. Both, ammonia and nitrite, are lethal to fish if they are exposed to high levels over prolonged periods.

      Read here to learn more about the nitrogen cycle.

  8. Linda Mulligan says:

    I have nine other Molly’s that have been in the tank for six weeks and the water has been tested.all good but my white molly just sits on its side at the bottom of the tank.he eats when I put food in and then seems like he’s gasping for air and just floats back to the bottom’s a 80 litre tank

    • Scott @ Algone
      Scott @ Algone says:

      I would remove and quarantine the fish. If the water tests safe, specifically for nitrite and ammonia, then it is quite possible that the fish is ill. Fish stress caused by toxic water weakens the immune system of fish making them more susceptible to illness, however diseases can also be introduced by adding new fish to the tank. The disease is not always immediately apparent. Your local fish store should be able to recommend treatment based on a description of behavior. But your immediate concern should be to safeguard the other fish by isolating the sick one.

  9. Thilo
    Thilo says:

    Hello Carol,
    Check the calcium levels, it could be calcium precipitation. Keep in mind that only pure water evaporates, and minerals are left in the water column. Replenishing water with saltwater will increase the mineral content over time. If you are using a phosphate remover, some are known to cause a discoloration if phosphate levels are too high, or the product is overdosed.

  10. Carol Crane says:

    I have been in the aquarium maintenance field for 40 years and haven’t ever seen this. Hope you can help. I have been fighting a white cloudiness problem for at least 6 months. this is a saltwater aquarium of 45 gallons. I have took care of this aquarium for 21 years and did regular water changes using water from the ocean. I have it shipped to me and it is very clear. The fish are extremely healthy and they are 21 years old The cloudiness doesn’t seem to bother them but it bothers me. I did a water change 4 days ago and the water looks like I have put a couple of gallons of milk in it, hardly see anything. The cloudiness never changes. The water has tested ok . HELP!!!!!!!!!!!!!!!!!

    • Thilo
      Thilo says:

      Hello Carol,
      Check the calcium levels, it could be calcium precipitation. Keep in mind that only pure water evaporates, and minerals are left in the water column. Replenishing water with saltwater will increase the mineral content over time. If you are using a phosphate remover, some are known to cause a discoloration if phosphate levels are too high, or the product is overdosed.

  11. jen says:

    Please help. I am in the process of doing a fishless cycle using pure ammonia. I am around 4 weeks into it with nitrites and nitrates off the chart. All of a sudden, I started getting a positive reading for chlorine/chloramine, and I haven’t added water. I have 2 existing tanks, established, which use the same water source and dechlorinator (Prime) with no issues. Why the sudden positive?

    • Thilo
      Thilo says:

      This is likely a false positive, check your test kit and make sure it is not expired. These kits are sensitive and easily contaminated.

  12. Rob says:

    We have had upwards of 50 fish die over the 6 weeks of various breeds.
    There were just over 100 fish in total in the 650L tank before we starting experiencing problems.

    Number of Deaths Fish Type
    1 Blue Ram
    1 Fighter Fish
    3 Angel Fish
    10 Guppies
    4 Platies
    0 Bristlenose Catfish
    1 Lace Gourami
    3 Honey Gourami
    2 Dwarf Grourami
    2 Blue Gourami
    10 Neon Tetra
    5 Rummynose Tetra
    1 Red Tailed Shark
    2 Albino Shark
    1 Silver Shark
    4 Clown Loach
    1 Redticulate Loach
    0 Black Widdow Tetra
    0 Kuhli Loach
    1 Female Gourami
    7 Sale Fin Molly
    1 Penguin Tetra
    1 Flying Fox Shark

    The tanks water parameters all test normal every time we do a series of tests
    0 ammonia
    0 nitrite
    0 nitrate
    7 dgH
    6 dkH
    7.0 to 7.4 pH depending on time of day

    We are injecting CO2 at a rate of about 5-6 bubbles per second in a 650L tank. The drop checker using a provided solution indicates green level of CO2. Light pearling of most plants occurs in the tank during the evening.

    The tank was going along great and seemed to be cycled based on our testing.

    We think the trigger for fish death has been utilising galvanised mesh (heavy metal poisoning) in the tank to separate some new angel fish from existing angels (the angels are going great and are getting along now with some minor chasing away happening). After removing the mesh, we did a 90% water change. This seemed to slow down the rate of death. We are still experiencing a fish death every week or so..

    I have not seen any overt aggression. Some of the species chase their own kind s from time to time but they seem to have generally worked it out andget along most of the time.

    Any advice would be greatly appreciated!!!

  13. Vernon says:

    My Algae eater was sitting on the bottom of the tank for a while, and I thought that he was eating the algae. Until he started to swim on his left side and turned upside down on the bottom of the tank. Then after that he moved his mouth only once then did not do anything.

    • Thilo
      Thilo says:

      You need to check the water parameters (ammonia/ pH) to see if those are an issue. Take a look at the gills, you are likely having some ammonia if they are red and inflamed. Increase the maintenance, do 10% water changes a week, vacuum the gravel and make surest to overfeed.
      If ammonia/ pH are not the issue, you could look at a distressed fish. Is the aquarium overstocked? Is the aquarium suited for the size of the fish? Lastly, it could be a swim bladder issue. So if all the above items are not the culprit, you might look at that disorder.

  14. DIANA L BROCK says:

    My male crown tail is lethargic , there’s scum on the surface and he’s hanging on the bottom. Is he sick? He ate, but won’t flare. He’s always been kind of lazy.

    • Thilo
      Thilo says:

      This could be ammonia poisoning. Please check your parameters to confirm. Use paper towels and drag them across the water surface to remove the scum. Oxygen enters the water by gas exchange at the surface.

  15. kyla says:

    hi guys my fish was in his tank and i forgot that you can’t add tap water and now he is not acting right he keeps floating to the top and gasping for air also tilting on his side what should i do??

    • Scott @ Algone
      Scott @ Algone says:

      You should check the ammonia and nitrite levels in your tank. Fish gasping for air at the surface of the water usually indicate ammonia/nitrite poisoning. This inflames the fish’s gills and makes it difficult for the to breath which is why they gasp for air at the surface. If you have detachable ammonia and nitrite levels you should perform about 20% water changes every 3 – 4 days to help lower them. Avoid too many water changes or larger water changes as this will only disrupt your biological filtration and make the problem worse. You can also consider feeding only once every other day during this time as the reduction in food will equal a reduction in waste and this will also positively contribute to lower nutrients and rebalancing the fish tank.

  16. Caitlyn Brittianna says:

    2 of my favorite guppies died, and they were having a hard time swimming forward and their tails kept drooping. What is this and is it contagious to 2 red wag platys, 2 more guppies, and 3 small neon tetras? Please help!

    • Scott @ Algone
      Scott @ Algone says:

      You should immediately check your water parameters for ammonia and nitrite. Both are highly toxic for fish and will cause death if exposed to either for long periods of time. Ammonia and nitrite poisoning may also cause disorientation and erratic swimming. It’s very difficult to diagnose fish health issues like this and we recommend you speak to a professional at your local fish store. Knowledgeable retailers should be able to help diagnose the problem and more importantly recommend treatment options.

  17. Gabibib says:

    Hi my betta fish is acting lethargic. Staying propped on leaves or tipped over on his side when he is resting in the gravel. He seams to be pulling lots of water across his gills and whenever he stops swimming he starts to float vertically. Yesterday while cleaning his tank i found a decoration that the paint had been dissolving off of. When i held it under the tap the paint just started to disappear. my two ottos seem to be lacking in their deep bro color but that only happened from chaining the water. Further a day before my betta got really lethargic I thought my ottos might have been mating because they were really active around each other and swimming on top of each other etc. I thought they might have milted causing the betta to be agitated. Initially when the betta swam he would swim normal and then jerk around. I checked for signs of ICk or velvet but didn’t see any. Any ideas on what it could be?

    • Thilo
      Thilo says:

      Labored breathing, inflamed gills, and a lethargic fish can be signs of an ammonia poisoning. Make sure that you are good on the vital water parameters, check ammonia without delay. Lack in color also indicates a below optimum water quality. There is also a possibility of an outside parasite, the most common in association of fading color, being lethargic and darting is velvet. You can check that by turning the aquarium and room lights off, then take a flash light to observe the fish. Velvet will be visible as a bronze/ gold dusting on the fish. Should this be the case, you need to medicate the aquarium.

  18. Jason Fruen says:


    Our family just got our first aquarium (54 gallons) and have started to stock it with glow fish and angel fish. Tonight I noticed the fish were all gasping for air. After doing some quick googling we decided to replace 25% of the water. We treated the new water for chlorine and gave it some stuff for good bacteria. We also turned on the bubblers and verified the water temp which was at 78 degrees.

    I think we added too many fish too fast. What should I do to try and help the fish further?

    • Scott @ Algone
      Scott @ Algone says:

      The aquarium has to initially go through a cycling phase for the biological filter to establish in the tank.Ammonia and nitrite are inevitable during this phase. Both of these nutrients are toxic for your fish and will inflame their gills making it difficult for the fish to breath. This is why they are gasping for air at the surface of the aquarium. To minimize the spikes of these nutrients you should feed very sparingly. As little as once every other day. Fish food waste and fish excrement is what is converted into ammonia, nitrite, and eventually the less harmful nitrate. Limiting food will limit the severity of the spikes. Also try to avoid large water changes during this time. It’s tempting to do because we want to help our fish, however if the beneficial bacteria are disturb it will only prolong the aquarium’s cycling phase (nitrogen cycle). You can also use Algone to reduce nutrients in the tank. Algone is a great additive for correcting imbalances in the aquarium as well as for preventive maintenance of a whole host of common aquarium problems related to poor water quality and accumulating waste.

  19. Allison Mays says:

    I have some guppies and striped loaches plus plates and swordtails. I gave them aquatic mites and I didn’t realise the mites died so quickly and I just fed the fish on a daily basis. I notice my fish are dying from the food. I know that because they die in perfect condition. Please tell me what to do!!! I have also gone back to giving my fish dry pellets.

    • Thilo @ Algone
      Thilo @ Algone says:

      Unless the mites have been contaminated, it is unlikely the cause of death. Fish should be provided with a complete diet (mix between dry, fresh, and live food) but it rarely causes fatalities. Please check your water parameters and make sure those are within the suggested parameters. Observe your fish behavior. Then take that knowledge to determine any issues that can led to what you described.

  20. Payton says:

    I’ve had my two goldfish for about 7 years now. We went from a 2 Gallon tank to a 5 gallon to a 10 gallon. We had the 10 gallon for about 4 year so it was old and started leaking. We went and bought a new 10 gallon with a new filter, and new gravel. I cleaned out the filter and the gravel, and the tank began doing that “new tank syndrome” thing where the new bacteria makes the water cloudy. However, one of my fish was gasping for air and floating in circles, dying. I moved him to new clean water as well as the other one and emptied the water and refilled it. Now, the dying one is in his own bowl without a filter, and the other is in new water, however it’s still REALLY cloudy, and I am worried he is going to gasp for air, and go through the same process and die when I can’t be there to help it. What should I do? Wait for the cloudiness to go away, and hope that he survives? Leave the other fish separate? Any ideas?

    • Scott @ Algone
      Scott @ Algone says:

      Hello, the cloudiness is not likely what’s causing the fish to gasp for air at the surface of the aquarium. This is likely the result of rising ammonia and nitrite. Both of these nutrients are quite toxic for fish and they will cause inflammation of the gills, making it difficult for the fish to breath. When you changed over to the new tank, the aquarium has to restart. The nitrogen cycle has to re-establish in order to break down waste inside the tank efficiently. During the cycling beneficial bacteria multiply until the tank is in balance. Unfortunately, ammonia and nitrite will spike during this cycling period. Once there are adequate colonies of beneficial bacteria present, ammonia and nitrite should drop to 0 and all that’s left are less toxic nitrate. Avoid feeding too much during cycling and also be sure to perform frequent water changes, every 3 days or so, of 10 – 15% of the water. This will help lower ammonia and nitrite, providing some relief for your fish.

      Please see the following articles for more info:

      The Nitrogen Cycle
      Beneficial Bacteria

  21. Jay says:

    I have a 50 gal tank that has no filter for over 5 years now. Recently the I put in a placo and after 3 days it died with when it looked fine the day before. When I took it out I noticed the water was yellow. What caused my placo to die? My tank also has 2 tiger Oscars that are fine but have been in the tank for about 2 years, are they at risk as well?

    • Scott @ Algone
      Scott @ Algone says:

      There could be a number of problems responsible for the placos death. It may have already been sick when you bought the fish. The change in environment may have stressed it too much and weakened its immune system. A filter also provides most of the oxygen in the fish tank by surface agitation, in other words, your tank may lack sufficient oxygen to support additional fish life. Airstones do not provide much oxygen as oxygen enters the aquarium through the water surface when there is adequate movement. See this article for more info. I’m not sure why you’re not running a filter but would advise using one. The yellow tint in the water are likely chemical impurities and should be able to be removed by running activated carbon in a filter.

  22. SVC says:

    I unfortunately used the wrong kind of silicone for Deco in my fish tanks and two bettas have been hanging on for a month now not eating and staying at the top or hiding at the bottom all the time now, not happy campers not sure what to do anymore. I have repeated salt baths for a week which seem to only delay the effects of fin rot which has returned after three different kinds of medications over three weeks. One finally died after being so stressed trying to hide in his sinking log, so I got a replacement for his buddy he left behind thinking it may perk him up- it did, but now the newbie is sick and it can only be the silicone i used to fix the log i put back into the tank. So my question is these guys are just hanging in there and I don’t know what to do is there a way to treat silicone poisoning in your fish it’s obviously something they’re going to be living with until they die but then are they really living theyre not even eating they’re slowly just wasting away. At least I have a chance with the new possibly its only been two days with him its been a month for the other.. after all the salt baths and water changes and gravel cleansing and medication treatment and pea treats different foods sunshine temperature fluctuations, do I just keep doing all that until he has nothing left in him or any thoughts on euthanization at this point?
    I can’t seem to find anyone who has fish that are still living with silicone poisoning and how to treat it or how their fish are living with it.

    • Scott @ Algone
      Scott @ Algone says:

      First thing to do is to remove the repaired log with the silicone on it. Next, I would perform large water changes of 40 – 50% of the water two or three days in a row. Avoid cleaning the filter at this point since you will risk losing beneficial bacteria resulting in an even worse situation. Obviously you shouldn’t add any new fish until you know your water is safe for healthy fish life again. The then that I can’t really suggest much more. As far as euthanizing your fish, the most humane way to do this if there are no other options is to put aquarium water into a small ziplock bag and place it into the freezer until it becomes slushy. Next, put the fish in the bag and continue freezing.

      Best wishes with the tank. I hope you are able to save your fish and won’t have to consider euthanizing them.

  23. Mirza Tarique HUssain says:

    I recently acquired a small aquarium, initially there were 10 fishes or so. The tank is around one month old. Then gradually out of excitement I added a few more ( angel fish, fighter, bubble eye, wisu gold fish etc) then after a week or so they started dying rapidly, I guess it due to nitrite poisioning. any other suggestions?

    • Thilo @ Algone
      Thilo @ Algone says:

      Your small aquarium might just have successfully housed the fish it was able to provide the proper environment for. Adding more fish causes the aquarium to adjust chemically and biologically. The beneficial bacteria converting ammonia to nitrates have to grow their colonies in order to process the additional waste. While this is happening, the breakdown of ammonia to nitrite to nitrate is slowing and ammonia/ nitrites levels will spike. This is the likely cause of your fish dying.
      Please consider if your aquarium is “large” enough to hold the fish you want, and if so, just add one or two at a time while monitoring ammonia/ nitrite.

  24. Andrew Compo says:

    I think I may have co2 poisoning in my fish tank. How can I verify this? The Rena filter started to act weird a few days ago. Seem to have a lot of air coming out. Now all my fish have died in two days. I tested the water and only thing was nitrates and nitrites were a tad high. What can I do to fix this filter. The Rena company is impossible to get a hold of