Slime, or algae, may look disgusting, and you certainly don't want your fish swimming in it, but it is a natural condition that occurs in water that is kept in an enclosed area. Keeping an aquarium clean and clear is not a natural condition, so it takes some diligence to keep your tank crystal clear.
Slime, or algae, can take over a fish tank quickly. To keep it in check, it is important to look at your tank daily. Make sure the water is clear and there is no sign of buildup on the sides of the tank or the rocks or other decorations inside. Wiping any buildup off as soon as it develops can be enough to keep the slime from taking over your tank. Just use an aquarium brush, or a regular household sponge, purchased new and used only in the aquarium, to clean slime. Do not use any cleaners or chemicals.
If the slime is well established, you can change the water in the aquarium. This removes dissolved food and other waste material that algae feed on. Do not make the change all in one day; instead, remove 20 percent of the water from the tank and replace it with fresh water each day until the tank is clean and clear.
Too much light can cause algae to bloom in the tank. If you are lighting the tank all day, or around the clock, start turning the lights off occasionally. You want to limit the tank to about eight hours of light daily. Daylight can encourage slime to develop as well, so, if your tank is located near a window, you may want to relocate it or install blinds so you can control the amount of sunlight it receives.
One common cause for the development of slime on the inside of a fish tank is overfeeding. What the fish do not eat remains in the water, gradually breaking down and providing a food source for algae.
Consider adding creatures to your tank that help clean. Snails and hermit crabs both feed on algae, and can do an effective job of maintaining a clean tank. They cannot, however, clean a tank already covered in slime, so add them to the tank after a good cleaning.
It is normal for things to wear out over time. This can happen without you realizing it. Your filter may appear to run fine, but need to be replaced. The heater may be slightly off and keep the water a little warmer than usual, which encourages algae growth. You may get careless about filtering tap water before adding it to the tank, which can also lead to slime problems. If you haven't had problems with slime in the past, look at any changes to the routine that may have caused the slime growth and make sure all of your equipment is in good working order.