Many heaters are not accurate and require monitoring to assure that the desired temperature is maintained. A filter of some type can also be quite useful. I prefer the fully submersible type that do not require an air pump, but are self-contained units, sucking water in through the bottom, passing it through a sponge filter, and expelling it out the top. For larger aquariums canister filters are a good investment. Although their initial cost can be high, the time saved in water and filter changes makes them well worth the money.
Many different models are available, from a number of different manufacturers.
As with aquariums, larger is usually better. The greater the volume of water moved and the larger the filtering surface, the cleaner the water will remain. This will reduce the frequency of water changes required. Undergravel filters can be used, but often become clogged too quickly due to the large amount of waste material that turtles are capable of producing. In my experience, hatchlings raised on a balanced diet with calcium and vitamin supplements do not require full-spectrum lighting or additional UV radiation.