How to Treat Smelly Aquarium Water

A diagram of how to clean stinky aquarium water

The Spruce

Unless you place your nose directly over the tank filter, you'll find that most aquariums are relatively odor-free. Occasionally though, aquarium owners report unusually smelly aquarium water. If you can locate a fish tank using only your nose, there is something wrong. Quick attention is indicated.

Causes of Odors

A well-maintained aquarium is not likely to ever develop an overpowering odor, as bad odors are usually caused by decomposing biological material (bio-waste) that has not been cleaned out. Uneaten food that is rotting underwater or excessive amounts of normal fish poop is often the culprit. But rotting plant material can also be to blame. Here are some sources of bio-waste:

  • Dead fish: The most common cause of a smelly tank is a dead fish. It may be several days following the death of a fish before the owner realizes something is amiss. It may have hidden in an out of the way corner of the aquarium and died, leeching proteins and oils that float to the surface and off-gas (evaporate). Promptly locating the body and removing it should eliminate the odor.
  • Excess food: Overfeeding is another common cause of bad odors from fish tanks. Uneaten food falls to the bottom of the tank, where it promotes an overgrowth of bacterial colonies. As the bacteria grow exponentially, they release waste gasses that have a foul odor.
  • Wastes from an overstocked tank: Excessive bio-waste can also be the result of overcrowding. Overstocking the tank means that too many fish are living in too small a volume of water. When fish eat, they produce excrement. As the number of fish increases, so does the fish poop. Eventually, the overabundance of waste is too much for filters and beneficial bacteria to process.
  • Decomposing plants: While this is less common than decomposing proteins and oils, a rotting plant even underwater can emit a horrible stench when left unattended. Luckily, dead plants are easy to spot. They often turn a slimy brown or black color and foul up the water quality with turbid, murky water.

Source Removal

The first thing to do is to keep a good running inventory of your fish. Are any missing? If so, start searching for the remains, as that may be the cause of the bad odor. Be aware that sometimes fish are consumed by their tank mates, so a missing fish may never be found, but predation won't cause an odor.

If a decomposing fish body is not the source of the odor, and all of your fish are present and accounted for, then the problem is likely due to a buildup of organic material in or on the gravel. Dislodge tank furniture to look for plant remains and particles of uneaten food. Use a tank vacuum or scoop to clean the substrate (ground material) either inside or outside the tank.

Give the whole tank a good cleaning, and then cut back the feeding regimen to one small feeding per day. It will also be necessary to clean the filter before and after the cleaning process. If there is a lot of debris in the tank, this indicates that the filter has likely been clogged and not filtering well for some time. After removing every source of bad smell, do a smell test again.

Preventative Steps

Once you’ve eliminated the source(s) of the immediate smell, wait for a few hours and use your nose to sniff again. When all smells are gone, there are some steps to take to ensure your tank keeps smelling clean.

Feed your fish sparingly. Unless you stop feeding your fish entirely, they will not die of starvation. In fact, fish can and will suffer a variety of disorders if they are overfed. 

Schedule regular water changes and tank cleanings. Every aquarium is a closed environment, so cleaning is an absolute must to ensure a healthy environment. Sometimes odors are the result of a slow rise in bio-waste by-products, due to never performing or skipping a water change.

When cleaning, don’t forget the filter. Use activated carbon media in your filter to help remove odor-causing molecules (ions), but note that the available surface area in the carbon does get used up and has to be changed a few times a year to remain effective.