If your fish tank smells like sulfur, or rotten eggs, it usually means you have a serious water bacteria buildup, a chemical imbalance, or a problem with your aquarium pump or filter. This type of environment is stressful and unhealthy for your fish; it needs to be taken care of as soon as possible. Letting the problem go for too long can make the condition worse; it can kill your fish.
Move Your Fish
Fill up a bucket with equal parts aquarium water and same-temperature dechlorinated tap water, and transfer your fish from the tank into the bucket using a net. If it's going to take you a while to clean your tank, put the bubbler hose from your tank into the bucket to make sure the water is oxygenated. Remove plants and decorations after you move the fish, putting them in a separate bucket for cleaning.
Drain the Tank
Use a siphon to drain all of the water from your fish tank and dispose of it by flushing it down the toilet. If you don't have a siphon, you can use a plastic cup or a small handheld water pitcher to remove the water. If you can safely move your tank outside to clean it with a garden hose, that's your best bet. If it's too big, use a water pitcher to flood the bottom of the tank and the gravel with fresh water and stir the rocks with a gloved hand or a long-handled wooden spoon. Continue siphoning and stirring the water, as you're going to be getting a lot of debris out of the tank.
Clean the Tank
Use a softer scrub brush to clean the inside of your tank walls, and use a razor blade to remove algae buildup. Continue flushing the tank with clean water until it runs clear. Move on to clean the plants, decorations and filter in the same manner, using a scrub brush and clear water. Refill the tank to its normal level using dechlorinated tap water. Allow the water to reach the appropriate temperature for your particular breed of fish, then add your plants and decorations into the tank before returning the fish.
Keep It Clean
Get into the habit of removing uneaten food so it doesn't clog up your filter or cause your tank to become so dirty that you get another imbalance that results in an unhealthy water environment and a sulfurous smell. Do a partial water change every week or two by taking out 10 percent to 20 percent of the water and replacing it with dechlorinated tap water. If you see algae building up inside your tank, use a long-handle scrub brush to remove it, or get an algae-eating fish like a plecostomus to help keep the tank clean. Talk to your vet about other commercially produced water additives that can safely help you cut down on smells in the future.
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