Turtles are heavy feeders and an overloaded container will be very difficult to keep clean. Red eared turtles will shred their food as they eat it, contributing to the waste load of the tank or pond. If desired, turtles can be fed in a separate container, thus reducing the amount of food left in the their living space. Remember that aquatic turtles must be in water to be able to swallow. It is also important to use water that is the same temperature in both containers.
If the tank is not overloaded, a well established under gravel filter, driven by a power head (the standard air lift just doesn't provide enough water flow) and with 3 to 4 inches of gravel bed above it, will maintain excellent water quality. The oxygenated water flowing through the gravel bed encourages the establishment of a colony of beneficial bacteria. These bacteria then aid in the reduction of waste products. Remember that it takes a few weeks for the aerobic bacteria to become well established in the gravel bed. Once established, constant water circulation is important. The bacteria colony will die if deprived of oxygen for an extended time. An occasional partial cleaning of the gravel bed using a standard aquarium gravel cleaning siphon is required to remove any undigested waste and to prevent the filter bed from clogging. This periodic siphoning also affords an opportunity to top up the tank with fresh water and reduces the buildup of harmful materials. Large power canister filters, although expensive, also work well. The filter media in the canister filter will have to be cleaned or replaced every one or two weeks.