Remember when cleaning your filter sponge never use tap water. Take a small bucket and fill it with tank water. Why? This will prevent you from killing the beneficial bacteria that are living in the sponge. Public tap water contains chlorine, chloramine, or a combination of both. These will kill the beneficial bacteria colony within your filter sponge. Remember beneficial bacteria helps break down fish waste and are part of the nitrogen cycle. You need these little guys to stay alive and healthy to remove harmful levels of ammonia from your tank water. Many times, a new hobbyist will make this mistake. In just a few days or weeks depending on the size of your tank, feeding practices and the number of aquarium inhabitants they begin to experience mysterious fish loss. Why? You have killed off most if not all the beneficial bacteria when running tap water over and through the filter sponge while cleaning it. So, in short you have started a brand-new cycling of your tank. So always remember to use tank water while cleaning your filter sponge to prevent losing your beneficial bacteria.
Darren & "Dulce The Happy Fish" 