Odorous fish tanks occur for several reasons that include rotting food, undiscovered dead fish or snails, dead plants and excess waste. Overcrowding of fish tanks can also lead to foul odors. Improper tank cleaning is a common culprit as well.
The first step to eliminating foul odors from a fish tank is to perform a thorough inspection for dead fish or critters and promptly remove them. The next step is to perform a 10 to 15 percent water change. It is not necessary to remove everything from the tank, but it is always a good idea to remove plants and figurines to ensure they are properly rinsed.
If the odor persists after cleaning, the next step is to replace the filter. Filters that are clogged with waste and debris can emit a stench even if the water is clean. A common mistake many fish owners make is to not follow the recommendation of changing the filter once a month. If odors persist after a cleaning and filter change, it is important to ensure that the fish are healthy, as stressed or diseased fish tend to emit a strong odor. Consulting with a local pet professional can help to pinpoint the problem.