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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I have a question. I just got a betta fish on the date of 9/16/18 and I bought 1 gallon water from Acme and used that for the water. I put the fish in the bowl and he’s very active, eats right away, and barely sits still but of course takes an occasional break. He swims beautifully but sometimes, when he swims he jerks his side almost like he’s about to take a tight turn, and then keeps going straight. I’m worried that he may be sick.


Aquariums need to cycle, which means the settling of bacteria colonies that process ammonia into less toxic nitrates. Follow this link to learn all about there is to know to make the aquarium safe for the fish:


One of my siblings dropped a small metal clip into my tank and i’m worried it could have poisoned my fish. I have 2 goldfish in a 20 gallon long tank and they are typically very active and always swim around when I come near the tank. The clip was in the tank for only a day or 2 but it seems to have a little rust on it. One of the goldfish is acting normal but my other on is not interested in food and is acting very lathergic. I am going to do a large water change to… Read more »


It is very unlikely that the clip has caused major issues for the fish, but you can use fresh activated carbon, and a water conditioner or a water softener that remove traces of metal. This would be the preferred initial step over large water changes.

carmelo lofaro

there are dark circular growths on the aquarium glass, my large catfish has died, what are they

It’s likely algae growth on the glass. You should check your nutrient levels. Ammonia and nitrite must not be detectable in a healthy aquarium. Both are lethal to fish after prolonged exposure. Ammonia and nitrite poisoning inflames the gills, making it difficult for fish to breath, resulting in illness and death if not corrected quickly. Since ammonia is naturally broken down into nitrite, and eventually nitrate, your aquarium will further be at risk of an algae outbreak. Nitrate is a natural fertilizer which readily available algae spores will feed on. If you find your nutrient levels are high, you should… Read more »


I have 4 gold fish in a 20 gallon tank. I made sure I did half water change weekly and everything was going great. My wife went to a yard sale and bought some fish equipment, some conditioners, icky medicine, and a few other items. She decided to put some of the conditioner and ick medicin in the tank before the water change. When I got home 2 of the fish were at the top up right not moving gasping for air. 1 of the fish was at the bottom not moving, and the other one darting around the tank.… Read more »

I think you did what you could with the whole tank water change. Your tank will likely need to re-cycle, so ammonia and nitrite spikes are to be expected. Both are highly toxic to fish if exposed for prolonged periods. This is however unavoidable during the cycling of the aquarium. A common symptom of ammonia poisoning is that fish gap for air at the water surface. This is due to inflammation of the gills caused by the ammonia. To reduce the stress, you can change about 20% of the water every 2 – 3 days during the cycling. Careful not… Read more »


Seeking your advice . An oscar fish swallowed a cryistal of sodium thiosulfate ( dechlorination cryistals) any problem expected and how to treat the situation.
Thank you

Not sure there’s a whole lot you can do since he already swallowed it. It’s probably best to contact the manufacturer of the dechlorination crystals with any concerns about your fish’s health after swallowing it.

Is well water what caused my crayfish to die?

You should check your water chemistry. Well water is generally not toxic, although we recommend using a water conditioner before adding fresh water to the aquarium.

Vicky Wolf

I also wanted to say that like algae fish I’ve had that fish for quite a while I’m not sure really what happened at all same with the neon tetras I’ve had those for a while nothing has been knew that’s been introduced adding some Sea aquarium salt so once I get home I’m going to try that and see if that helps of course it’s not going to revive it a dead fish or almost dead I love watching these fish and my cats enjoy it too but I hate to see them dying it’s just disrupts me and… Read more »


Can you help me? My Pictus catfish is acting very weirdly, he swims to the surface of the water and gasps for air, and then falls back down like he’s dying. he breathes very quickly and swims upside down. hes always lying down on the tank on it’s side and it keeps making me think its dead. I removed it from the tank, which consists of five Zebra Dainos, one Red Eye Tetra, and a Chinese Algae Eater, and put it in a seperate tank. I smelled the water, but it smelled very foul. I’m pretty sure that’s the reason… Read more »


First, lets make sure the water is within tolerances. Check especially on nitrite and ammonia. Next, increase oxygen levels by adjusting the filter outlet to create a disturbance on the water surface. It might be stress. Is the tank large enough? Lastly, feed a pea to the fish. It is known to help with swimmbladder disease, not suggesting that the fish has that problem.


Thank you for the info though my picture cats have already died I was very sad to see him lying motionlessly at the bottom of his quarantine tank. After this, I decided to quarantine the rest of my fish and do a full water change and wash all the decor and gravel, but don’t worry I made sure to add the required beneficial bacteria and cycle my tank. I bought an ammonia, ph, and nitrite testing kit. When everything was okay, I added my fish and nothing has gone wrong since. Sorry for replying so late. Thank you for the… Read more »


Ok so I told my dad that my fish tank is getting dirty and asked him if he could help me clean it. But he keeps saying tommorow. So I told him it needs to cleaned now. So my question is will a dirty tank kill my fi sh? I have a ten gallon tank with five small fish. But after it got a little bit dirtier I noticed that one of my fish was turning white, it was originally orange with brown spots. But now it’s white, and floating, but not dead?! So do you think you could help?!… Read more »

It doesn’t sound good with your fish. Best “cleaning” practice is to use a siphon to remove water while poking around in the gravel. This is also called vacuuming the gravel, since doing this will remove waste particles that settle at the bottom of the fish tank. My advice is to talk to someone knowledgable at the fish store. Ask them to show you aquarium siphons and have them explain how to use it. In case you’re not already familiar with it, it’s really just an inexpensive hose with a larger tube attached to one end. When using a siphon,… Read more »


If anyone can give me some feed back I’d appreciate it! I recently bought a 20 gallons used. So I thought I should clean it up with the it full of water and add some vinegar. I rinsed it out once or twice and then put warm water 85 degrees or so, and I put water conditioning and starting liquid and put my fish in I bought 2 hours or so after I set the tank up and acclimated them for about 15 minutes or so. They died he next day. The water after I took the fish out went… Read more »


The bubbles are not the problem. These are either cleaning supply residue, or protein that causes air bubbles to get caught (quite similar to the foam in protein skimmers). The most likely cause for your fish problem is ammonia or a very low oxygen levels (due to water surface tension/ film on the surface that prevents oxygen to enter the aquarium). Do you have a filter? When setting up an aquarium, certain bacteria need to get established to make the water safe for the fish. This can take a few weeks, and during the cycle ammonia and nitrite will spike,… Read more »


So my little brother pored almost a whole bottle of ick medicine into the fish tank when i was at school. Will that effect my betta, from to much medicine, will he be okay if i keep him I that tank? And he is being treated for ick currently but will it hurt from to much?

Yo should contact the manufacturer of the ich medication. They should be able to tell you what action you need to take.


I hope someone can help! I have a 20 long heavily planted with fluval stratum dirt. I have a lot of platies, 2 angel fish (small), 1 dwarf gourami, 1 amano shrimp and 2 otos. I cecked the perameters and everything was fine but 1 fish keeps on dying each day… Someone please help.


Sorry we need more information to provide an adequate reply. Please contact our customer service department with more info. We do look forward to being of assistance.


I hope you can help! I had a Marineland Portrait 5 gallon set up since last summer and had 2 guppies happily living in it. I do regular maintenance, water changes, and water tests. Everything is always good. All of a sudden last week they were acting strangely, hanging out near the surface. I checked the water- no ammonia, no nitrites. I did a small water change anyway, just in case. Overnight 1 died and the other died the next morning. A couple days later I cleaned everything, emptied everything out, but thought it best to leave the filter with… Read more »


The signs could be interpreted as ammonia, or nitrite. With relatively small aquariums fluctuations are much more likely as in larger setups. It could also be a pH fluctuation. Just for good measure, check the expiration date on the test kit.
Be careful while cleaning the aquarium, this can easily be overdone. Only rinse out the filter with conditioned water.


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!!!


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


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!!!


This is probably stress related. Check for physical injuries and make sure your water parameters are within tolerances. Especially ammonia and pH. You should not clean daily, especially filter and gravel where the beneficial bacteria colonies settle.
We have a maintenance schedule published online that you can use.


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??


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.… Read more »


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 ?

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.


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.

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… Read more »


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… Read more »


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.


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

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.