Household bleach is an effective disinfectant, able to kill most bacteria and fungi. It's also highly toxic to fish and the microorganisms they depend upon. Obviously, you cannot use bleach to clean an occupied aquarium. It can, however, be used to disinfectant aquariums and aquarium accessories, provided you are careful.
When to Use
Bleach should be the absolute last resort, when you need a disinfectant and have no alternative, not part of your normal care routine. You’d use it on a second-hand aquarium, in a tank where fish have died from disease and to sterilize accessories that cannot be boiled or baked. Plastic accessories normally cannot be subject to extreme heat, because they can become distorted or melt. Large pieces of wood might not fit into a pan or oven, in which case bleach is the only option.
Sterilizing Aquarium: Preparation
To sterilize an active aquarium, take it down. For a saltwater aquarium, this requires preparing a second aquarium several months in advance. Your remaining creatures must be moved somewhere, and the only safe option is a fully cycled aquarium. The same goes for a freshwater tank, although the cycling process won’t take so long. For a second-hand tank or one in which all the animals have died, you just need to sort the accessories. Place gravel and rocks in a large glass pan of water for boiling and put all plastic ones to one side for bleach sterilization or disposal later.
Sterilizing Aquarium: Procedure
Use plain chlorine bleach diluted with water to make a 10 percent solution. Other household cleaning products and disinfectants are not suitable as it may be nearly impossible to remove all traces. Sponge the solution onto the inside of the tank and wait at least hour. For the treatment of serious diseases, fill the tank with a bleach solution and leave it overnight. Rinse the tank several times to remove almost all the bleach and then leave it exposed to air for at least a day, preferably longer. This allows the last traces of chlorine to dissipate into the atmosphere.
The accessories in an infected tank must be sterilized as well as the tank. Boil the dead rocks and substrate for at least 20 minutes and allow to cool. Dip plastic accessories into a 10 percent bleach solution and rinse them thoroughly. Rather than risk bleach, you might prefer to throw out these items and buy new ones. Found decorations, such as driftwood, may be carrying a multitude of pathogens and parasites. Boiling usually works well, but in the case of large pieces of driftwood, you’ll need to spend time soaking them in a bleach solution and then soaking out the bleach, a process that takes weeks. Bleach will, obviously, kill the organisms in live rock -- there is no way of sterilizing it, although it could be quarantined.
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