A bad odor in your fish tank could be caused by any number of things, including a hidden dead fish, a tank that needs cleaning or too much food being added at feeding time. Finding the cause of the bad smell is the most important step in eliminating fish tank odors.
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Finding the Cause
Start by removing any plants or decorations that a dead fish could be hidden in. Shake them vigorously to dislodge any dead fish or debris. Check also for an accumulation of uneaten food under or behind decorations. If you are using a carbon filter, check the cartridge to ensure it does not need to be changed. For all other types of filters, take the filter apart and clean it thoroughly.
Keeping it Clean
Keeping your tank clean is very important, not only to keep odors away but also for the health of your fish. The best way to maintain a clean tank is to vacuum the gravel at least once a month and to wipe the algae from the walls of the tank as well as the decorations. While doing the monthly vacuuming, you will remove 30 to 40 percent of the water in the tank and replace it with fresh water. Remove 10 to 20 percent of the water once a week and replace it with fresh water.
Adding too many chemicals to your tank can cause odors. It is best to use a bare minimum of chemicals by using only a water dechlorinator and a plant food for live plants. Use other chemicals only if they are necessary for treating illness, or for fixing major issues, and then only as long as needed.
Overpopulation and Overfeeding
If there are too many fish in your tank, there will be a constant odor because too many fish will produce more waste; even following a cleaning schedule will not keep the odor away. In general, there should be no more than one inch of fish per gallon of water. Less is better because it allows room for the fish to grow.
When feeding, add only the amount of food that will be eaten within two minutes. If any food is left after two minutes, reduce how much food you drop in to the tank. Leftover food sinks to the bottom and causes bacteria and odor.