What to do if your goldfish tank smells bad

In the wild, fish live in large bodies of water such as lakes, rivers, streams, and oceans. These bodies of water are so large that the conditions within them are much more stable than our small ponds and goldfish tanks. This makes them more able to cope with environmental changes or upsets within one part of the system.

It’s important to keep the balance of the tank and everything that lives within it just right. This prevents a range of problems within the tank that can manifest themselves as a rather nasty stagnant or fishy smell.

Does your tank smell bad? Here are some reasons for why this might be, and how to rectify or prevent it…

How to get a nicer-smelling tank!

Waste breakdown

Goldfish produce a lot of waste and it is important to provide adequate filtration to account for this. Fish feces is rich in ammonia, which can soon cause bad smells within the tank, as can decaying food left uneaten on the floor of the tank.

If your tank is beginning to smell then you should check that your filtration system is up to the job, and make sure that you’re cleaning out the tank and performing water changes regularly enough.

Is your tank too clean?

At the opposite end of the spectrum, a tank that is cleaned too often or too vigorously might begin to smell as well!

All ecosystems rely upon the presence of good bacteria within the water to kick-start the natural process of recycling and breaking down waste. Many important bacteria and enzymes dwell in the filter of your tank  and the growth of these should be encouraged in order to help the tank to cycle properly.

If you clean the tank too often, wash these good bacteria out of the filter, or remove them altogether by replacing all of the filter material, you may find that your tank will smell until the balance has the chance to right itself again.

You should avoid replacing all of your filtration materials at the same time and allow the natural lifecycle of your tank to establish itself.

Changing out too much water

In an established thank with stable water conditions, you shouldn’t have to change more than 20% of the water at one time. It is not uncommon for goldfish owners to remove 50% when cleaning the tank, however, and this can lead to upsets in the tank’s bacteria levels.


If you have had cause to medicate your fish with a water additive, these usually work by attacking parasites and bacterial infections within the tank and on the fish, but again, may kill the good bacteria!

If your tank begins to smell shortly after your fish have been treated for illness, this is almost certainly the cause, and you will need to allow time for the balance of the tank to return to normal.

Leave a Comment