Determine the Sources of Smelly Aquarium Water and Treat Them

A diagram of how to clean stinky aquarium water

Most aquariums are reasonably odor-free until you position your nose right over the tank filter. However, aquarium owners occasionally report extremely odorous aquarium water. If you can detect a fish tank just by using your nose, something is awry and immediate care is required.

Causes of Odors

Bad scents are mainly created by decomposing biological material (bio-waste) that has not been cleaned away, therefore a well-maintained aquarium is unlikely to develop an unpleasant stink. The culprit is frequently uneaten food decomposing underwater or excessive volumes of typical fish waste. However, decomposing plant matter might also be to fault. Here are several bio-waste sources:

  • 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. The fish 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: 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 step is to keep an accurate inventory of your fish. Are there any that are missing? If this is the case, begin looking for the remains. Remember that fish are often eaten by their tank mates, so a lost fish may never be recovered, although predation will not leave an odor.

If the odor is not caused by a rotting fish carcass and all of your fish are present and accounted for, the issue is most likely caused by a buildup of organic debris in or on the gravel. Dislodge tank furniture to search for plant remnants and food particles. Clean the substrate (ground material) inside or outside the tank with a tank vacuum or scoop.

After cleaning the tank thoroughly, reduce the feeding schedule to one modest meal each day. Before and after the cleaning operation, the filter will need to be cleaned. If there is a lot of debris in the tank, the filter is probably blocked and hasn't been filtering well for a long time. Repeat the smell test after eliminating all sources of foul odor.

Preventative Steps

Once you’ve eliminated the source(s) of the immediate smell, wait for a few hours and 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 frequent tank cleanings and water changes. Because an aquarium is a confined habitat, regular cleaning is essential to maintain a healthy ecosystem. Occasionally, scents are caused by a gradual increase in bio-waste by-products as a result of never executing or skipping a water change.

Remember to clean the filter as well. Use activated carbon media in your filter to assist remove odor-causing molecules (ions), but keep in mind that the carbon's accessible surface area wears down with time and must be replaced a few times a year to stay effective.