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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Hello everyone I have a question for any experienced refers out there. Can improperly rinsing carbon before adding to filter kill fish? I added two 100g packs of fluval carbon to my filter. Instead of rinsing them in my sink like I normally do, I took about a gallon of tank water and swished them around until the water turned black then added to my filter. I did this at about 5pm. By next morning all but my clownfish were dead and the clownfish was in bad shape gasping for air. I noticed the water also looked a bit cloudy… Read more »


Carbon is not likely the culprit, because fresh carbon does not release any potentially harmful substances. Fish gasping for air is a general sign of nitrites. As you replaced the carbon, the biological filtration might have been interrupted ever so briefly, but enough to create a temporary spike in ammonia/ nitrite.


I’ve been working on my levels very carefully for about 4 weeks, testing every other day, trying to cycle correctly. 20% water changes about once a week. All of my levels look good, ammonia 0, nitrate 20ppm, nitrite 0 , but PH was high 8.2-8.4. So I started using ph lower, treating once a day, over several days I’m at 7.8 I want to go lower but… My guppy went crazy a few days ago. He had jumped and landed on an inlet just out of the water and was there for only a few minutes before I found him.… Read more »


HI I have a reasonable sized pond outside with much tree cover, I had 26 beautiful goldfish starting with only 8 swimming around and very used to me coming and feeding them, we have recently cleaned the pond out by flushing it through with bore water (quite irony) and metalic tasting to our taste buds , just clearing the water so we could see our goldfish. today when I went to feed them I only have 6 goldfish left in the pond, no sign of any deaths, not sure where they have gone. any ideas one of the little goldfish… Read more »


I am sure you will find those fish upon closer inspection. Well water needs to be tested to be conclusive, but a metallic taste is likely not the healthiest for the fish. Most conditioners will remove traces of metal, and worth an attempt to safe the other fish. Again, test the water to see what else needs to be addressed.


Most likely birds are eating them, I don’t know what birds you have in your area, but here we have the Egret and they are excellent fishermen.


You cleaned the water so the predators could see their prey…


I did a 10% water change in 55 gal tank and a Roseline Shark died within about 30 minutes. This has happened twice when I have changed the water. The other fish are unaffected. I’m using Prime to remove Chlorine and a Neutral Regulator to adjust the Ph in the new water before I add it. I also add about 1/4 teaspoon/ 5 gal of Equilibrium to maintain the GH for my plants. I’m using injected C02 as well but in small ppm…maybe 15 ppm. KH and GH are both about 3 degrees. PH is 6.8 I can’t figure out… Read more »


The kH should be 4.5 of higher, lower levels in combination with CO2 can cause a pH shock. CO2 lowers the pH so the kH needs to be higher.

Ronald Shumaker

I bought a catfish last week and it seems to be gasping for air. I don’t know whats wrong with it. I have two of them and 16 other fish, they’re all doing good except this one catfish I got. It hasn’t ate, it lays at the bottom of the tank, when he tries to swim he sinks back down to the bottom. It’s like he has no energy. What could be wrong with him? I’ve put conditioner in it and some gravel cleaner, water clarifier in, and it doesn’t seem to be helping. I also changed out 4 gallons… Read more »

Thilo/ Algone

Fish can adapt to less then optimal water conditions because these changes take place over time. A new arrival is not given the time and therefore has a harder time. It could be shocked by introducing ot too fast to the main tank. Generally if a fish hovers near the bottom or gasps for air at the surface it can indicate ammonia/ nitrite problems. Loosing color and appetite are not good news. The fish might have been subjected to stress at the store, during transport from the wholesaler to the the store, etc. You might be able to safe the… Read more »


Hey so im concerned about my fishes that they keep on gasping for air. My mom today cleaned the tank when i was at work so she didnt know you could not clean the fish tank with soap. So she washed it and threw out all the water out of the tank and put the fishes in a bowl with sink water. After cleaning it she put the pebles back in the tank and put the bowel where the fishes were at, and added more sink water to the tank more then half the tank. She then turned on the… Read more »

This is not a good situation. The fish are likely gasping because of high ammonia in the water. If some soap actually stayed inside the aquarium when she added fresh water back to the tank, it would have killed off beneficial bacteria needed to convert waste. This would explain a spike in ammonia. Ammonia is highly toxic and will inflame the gill of fish, making it difficult for them to breath. This is why they gasp for air when ammonia is present. Another problem could be chlorine in the tap water if you have city water. The water would need… Read more »


I think I may have accidentally killed my shark by treating the tank with pure levamisole hcl powder. I don’t even think he was sick. But because I had the illness in my other tank and how highly contagious it is, I was advised to treat my other 40 gallon with only a shark. He seemed bothered by the meds right away and I should’ve realized the importance of his behavior but I didn’t at the time. Right after putting in the meds, I covered the tank with a blanket because the directions say to keep all light out of… Read more »


Levamisole is generally considered safe to use, but you should consult the manufacturer to find out what exactly happened.


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.

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