So your fish mysteriously died last night and you don’t know why. It could be one or several of the 21 reasons below.
Think about something you may have done that you didn’t do before. That could be the cause or maybe it’s just that you’re not taking care of your aquarium. In any case, the cause of your fish dying is most likely found below.
I start with the obvious things and then move on to the not so obvious.
Poor or no filtration: Too much ammonia or nitrites
If your filter is not large enough to cycle the water in your tank or if you’re not cleaning the filter regularly, you will have problems with excess ammonia and/or nitrites in your water. Too much will kill your fish eventually.
Make sure you have the right size filter. Clean it regularly (except for the bio media) and perform regular water quality checks for ammonia and nitrites. Test the water regularly.
Sudden change in water temperature
Maybe the heater malfunctions or the power goes out and suddenly the water temperature drops drastically. The opposite can occur if the heater malfunctions and the water gets too hot.
Either situation is not good for your fish. A sudden change down can severely stress the fish if they are the type of fish that can’t survive in colder temps. A sudden change up means less oxygen in their water and most fish need oxygen in the water to survive. Fish don’t have body heat like humans, so it’s up to us to regulate that for them.
Make sure you have a backup heater in case one malfunctions you can replace it right away. It’s also a good idea to have a separate thermometer to read the temps. Don’t rely on the heater-thermometer.
Sudden change in water pH
You may bring home some new fish in a bag and then dump them into your fish tank thinking everything is alright and then within days they die. It could be that the pH was much different than they are used to. You have to acclimate new fish to your fish tank first both for water temperature and pH.
Untreated tap water
City tap water contains chlorine and chloramine to keep down the bacteria for human consumption. Excess chlorine can kill the good bacteria needed for proper filtration and eventually kill the fish, so when you do water changes, you need to use a water conditioner specifically for chlorine and preferably one that treats chloramine as well.
Another thing to be careful of is when you go to clean your filter and other aquarium equipment. Make sure and use the aquarium water to do the cleaning and not tap water.
No aquarium heater
Not using an aquarium heater is fine for certain species of cold water fish, but most tropical fish can only survive in higher water temperatures in the 70-85 degree range.
Lack of oxygen in the water
As the temperature of the water gets hotter and hotter, less oxygen will be present for the fish to breathe. This can happen if an aquarium heater malfunctions or the fish tank is located near a window with direct sunlight or there may be a heat wave with no air conditioning.
Certain species of labyrinth breathing fish like betta fish may be able to survive for a while, but this could be a serious issue. Have a separate thermometer that constantly monitors the water temperature so you can address the issue right away.
The problem could be not enough water flow or poor filtration. It’s also a good idea to keep the water circulating with an air pump or powerhead to create some surface agitation and water circulation. Make sure your filter is up to the task and you regularly change out the mechanical and chemical filters.
Didn’t cycle the new fish tank long enough
For every new fish tank, you have to establish a nitrogen cycle. If there are not enough good bacteria in the biological media, your filter can’t absorb the harmful ammonia and nitrites. This is known as ‘new tank syndrome.’ It’s important to get the nitrogen cycle established and test the water until the ammonia and nitrite level is zero. Constantly test the water.
Overfeeding the fish
You should only feed your fish what they can consume in two minutes. Overeating is bad for us and for fish as well. They get fat and their organs start to suffer and disease may set in. Another byproduct of overfeeding is that the uneaten food waste settles in the substrate and as it decomposes it causes harmful ammonia to be released.
Once a day feeding is usually sufficient, but two smaller feedings could work too. The only exception is if your fish are herbivores meaning they are strictly vegetarian. Their stomachs are smaller so they need 4-5 feedings a day.
Fish tank too small
If the fish tank is too small, there is less water for the fish, which means waste can build up much faster and foul the water. When selecting fish tanks, bigger is usually better because it’s easier to keep the water clean (unless you overcrowd the aquarium).
Certain fish like to have lots of room with hiding places. If they are in a small tank, they may get stressed and eventually die. Do a little research first and find out about the fish you plan on getting. What are their habits and preferences?
Unsuitable fish food
There is a lot of cheap fish food out there filled with phosphate and other junk. Feeding your fish a steady diet of this is like you eating fast food all the time. It’s just not good for them. At least if you are going the fish flake route, make sure it’s decent quality food with very little filler.
Most fish especially carnivorous fish like a variety of foods. Frozen bloodworms, krill, or shrimp are great. Spirulina pellets are great for herbivores and omnivores. Do some research on your fish and find a good diet for them.
Disease and parasites
Watch your fish very closely. If they start to show signs of weakness like hanging out at the bottom or top of the tank a lot more, turned to the side or just not swimming like they used to or rubbing themselves up against decorations and substrate. You may start to notice white specks on their skin or the fins are starting to look raggedy, it could be Ich or Fin Rot. If they become bloated, it could be signs of Dropsy which is a more serious disease not easily treated.
Make sure to have some treatment on hand like aquarium salt and Melafix for parasitic infections so you can treat them right away. If possible, you should have a quarantine tank setup to separate the diseased fish from the others so it doesn’t spread. If you don’t have a quarantine tank, do a water change and add some aquarium salt as well as other treatments depending on the disease.
Improper water change
Never do a 100% water change and remove the fish. You should be doing a partial water change every other week with the fish in the tank. A 100% water change is too much of a shock for the fish, especially if it’s not treated and the temperature and pH are all off.
A more appropriate percentage would be about 25% because the water parameters won’t change near as much and it’s not so much of a shock for your fish. Before adding the water back into the tank, make sure the water temperature is close to what the tank temperature is. Also, make sure the new water is treated for chlorine if it’s city tap water.
Waiting too long between water changes
Even if you have good filtration, you still need to do partial water changes regularly. The reason is that while your filter system should remove the ammonia and nitrite toxins, you still have nitrate to deal with. Now nitrates are not as harmful as the others, but could weaken your fish over time if levels are too high. If you have a lot of live plants, it’s not so much of a problem because healthy plants will absorb much of the excess nitrates.
Another reason is that over time, essential minerals in the water will settle and become not effective at regulating pH. Over time, your tank could crash and the pH level could drop dramatically. That could be deadly for your fish.
You should be changing out the water with partial water changes about every other week or so. Some get away with once a month, but it depends how many fish you keep with the size tank you have.
Fish already diseased or sick
You may have purchased new fish from a pet store and not realized that the fish were already diseased. A lot of these places have poor conditions for the fish. They will overbreed them, feed them cheap food or use tons of antibiotics to keep the fish alive. With all the other duties they have, the store clerks just don’t have time to properly take care of them. Certain store chains use a centralized filtration system on all of their aquariums so disease can spread more easily to the other tanks.
Go to places that specialize in tropical fish. You may have to spend a little more, but it’s worth it to know the fish are of higher quality and have been cared for better.
Using detergents to clean a fish tank
Using detergents like Clorox or even dish soap is a big no no. Any residue left over will kill your fish pretty fast. Instead you’ll want to use something like vinegar or bar keepers friend to clean the tank itself if there is a buildup of calcium and other stains.
Overcrowding your fish tank
Too many fish, too little room, too much fish waste, too many ammonia spikes, too little oxygen for them equals stressed out fish. Makes sense right? When it comes to fish tanks bigger is usually better because it’s much easier to manage the water quality. Fish like any other pet, need to have some room to roam and establish territories. If you don’t provide that, it will be stressful for them.
So what exactly is overcrowding? Well, it’s not so cut and dried. It kind of depends on the type of fish you have, size of the surface area, and how large your fish will grow. Schooling fish like rasboras or barbs will require more room. Goldfish tend to produce more waste so you need to take that into consideration. Bettas are very territorial and tend to fight, so the fewer the better. The point is to do some research on recommended stocking for each type of fish. Equally important to know is how big they will get.
Another factor is the size of the surface area on your fish tank. The larger the surface area on your fish tank, the more oxygen exchange will occur. That’s a good thing. A tall, narrow tank has a much smaller surface area than a wide rectangular one so you can’t stock as many fish in a tall tank.
Using air fresheners
The jury is still out on whether or not this is true, but to be on the safe side, make sure your fish tank is covered if you spray air fresheners. Instead of spraying, you might try an oil-based freshener or candles or plugins well away from the aquarium.
Using flea spray around the fish tank
Flea spray is super toxic to fish, so you probably want to do this outside. If you do spray inside, you should tape some plastic around the top of the fish tank first to make sure none penetrates. You could also spray in another area or level of your home. Ant and roach spray can also kill your fish, so be wary of any potentially toxic things you may use in your home and not even think about.
Loud stereo speakers or other loud equipment next to your fish tank or even slapping the aquarium real hard can cause severe stress to your fish. This is because fish have a lateral line sensory system that help them detect changes in water pressure. Any sudden change can be a shock to their system. Any loud noises terrify fish and cause stress.
Using hand soap or sanitizer
You may not even think about it, but if you use a hand sanitizer or perfume or mosquito repellent or suntan lotion and then dip your hands in the fish tank. There are a lot of products like these that can be lethal to fish so you want to make sure that you are not dipping your hands into the tank after applying anything. Try and use a fish net or tongs or some other type of instrument to do what you have to do. Only use tools that are specific to the aquarium and haven’t been in the dishwasher.
Excessive cigarette or cigar smoke
Some fish are more sensitive than others, so if you’re a heavy smoker and your space is not well ventilated, then you may want to keep a hardy breed of fish like goldfish or danios. They’re not so sensitive to smoke toxins. It never hurts to have a cover over the tank as well.
It’s advisable to test your water pretty regularly for toxins like ammonia, nitrites and nitrates to see where you’re at with water quality. Also monitor the water temperature.
Sometimes your water could be fantastic with no nitrites or ammonia, but you’re fish are dying anyway. At that point you’ll have to do some investigation and think of things that you may have done recently like spraying or you changed the food or the water or put your hands in the tank. If you changed something that would be the first clue as to what may have caused the deaths. You may not even be aware of some of things that can be fatal to fish.
Let me know below if you’re guilty of any of the above.
When it comes to fishkeeping, what is your single biggest challenge or frustration? Leave a comment below.
This site is owned and operated by PSK Enterprises LLC. PSK Enterprises LLC is a participant in the Amazon Affiliate Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com.