Why Do Fish Die After Cleaning Tank?

cleaning sponge

Regular maintenance of fish tank is a integral part of responsible aquarium hobby. What baffles many hobbyist is that after cleaning tank and water change, they find their beloved pet fish dead. The only purpose of cleaning tank is to make fish tank a thriving habitat for fish but instead your fish life is cut short. There are many assumptions and misunderstanding about what might have led fish to their tragic end. In this post we will shed light on the real reasons behind your fish dying off after cleaning tank.

Routine Water change

Routine water change is crucial for keeping water tank habitable for fish but it is not uncommon for fish to die off after water change so what could be the reason?

You need to do partial water changes regularly( once every 10-14 days) to control ammonia and nitrate build up in aquarium. Added benefit of routine water change is that it also oxygenates water tank and removes any excess medication.

Volume of water change

To avoid tragedy first thing you need to remember is do not do large water changes. It can interfere with nitrogen cycle (you might kill off beneficial bacteria with large water change ).It is recommended that when you perform water changes, make it 10-15 percent of total volume of aquarium water.

Shift in water chemistry

Make sure that replacement water has similar water makeup like remaining water of fish tank because when water makeup is altered suddenly, fish won’t be able to adapt and will get stress. Some fish may die immediately and some may die within weeks or months due to shock.

Consistent water temperature

Do not introduce water that is cooler or warmer than the water in the tank. Ensure that the temperature of replacement water is same or within 2 degree Celsius or 3.5 Fahrenheit of remaining water. Sudden shift in temperature will shock fish and potentially kill them.

Dechlorinate water

Remember to treat replacement water with correct dose of dechlorinator before pouring it into the fish tank. Tap water contains chlorine or chloramine that are highly toxic to fish and aquatic plant. Read more about how to dechlorinate tap water here.

Accidental Contamination

If you found your fish dead after cleaning tank, there is possibility that you may have unknowingly introduced toxins that are deadly to fish. For instance

Did you reach into fish tank without washing hands?  Because germs and dirts or lotion in your hand can contaminate tank water. If you did wash your hand – were you thorough enough ? Because any traces of soap in your hand can kill fish in the tank.

Did you use soap or detergent to clean fish tank? They are very harmful to fish and should never be used for cleaning fish tank.

Did you use regular household cleaning sponges to clean glass tank ? They have chemical residues that can easily kill aquarium fish. Instead of regular household cleaning sponges, use algae scraper pad for cleaning inside glass.

If you had also cleaned aquarium decoration, did you make sure that they were aquarium safe before re-introducing them back into the tank? Did you properly rinsed off bleach before putting it back into the tank? Because bleach is deadly to fish. Read How do I disinfect my fish tank decorations? If you use toothbrush for scrubbing decorations then it is very important that you do not use it for any other purposes to avoid contamination.

As you can see there are more than one ways to accidentally contaminate fish tank. Any tools you use for cleaning should be solely dedicated to your aquarium only and always store them separately. If you use same bucket for cleaning fish tank and cleaning windows, floors or car, there is high possibility that your fish will come in contact with chemicals that are toxic to them.

Loss of Beneficial bacteria

Your fish will die after cleaning tank if you didn’t maintain good bacteria while cleaning it.

So extra precaution not to kill off beneficial bacteria because they breakdown fish waste, uneaten food and other organic debris in tank and keep water clean. Without beneficial bacteria, there will be spike in ammonia and nitrite level. Both of these are highly toxic to fish even at low level.

Beneficial bacteria live in filter – therefore when you clean it don’t wash the bio media with tap water. Tap water contains chlorine (or chloramine) that will kill all the bacteria in filter. Use bucket to fill in tank water and use that tank water to rinse your filter and to preserve good bacteria.

Beneficial bacteria are also found in substrate, rocks, plants and aquarium decorations.


The whole cleaning process can be very stressful for fish. Water changes if not done correctly will not only stress fish but will kill them also. Rearranging of aquarium decoration and disturbance to tank while cleaning will equally stress fish. Stress is one of the primary cause of fish death because it weakens their immune system.

So, if you don’t want your beloved fish to die after cleaning tank, take every step to make it as less stressful as possible for fish.

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