Although you filter keeps your water clear it only helps with the process that is really removing the waste from your tank. Biological filtration is the process where bacteria, convert the waste produced by the turtle into plant food. Knowing how this works will help you understand your water quality.
In simple terms, bacteria will colonize that will convert the waste into ammonia. During this colonization, your tank may get hazy, but this will go away on its own. Ammonia is toxic to animals that breathe in the water, but another bacteria, eats the ammonia and converts it to nitrite. Nitrite is still toxic. Fortunately other bacteria, will consume the nitrite and turn it to nitrate, which plants like algae will feed off.
You will help this process, by doing partial water changes to remove solid waste build up in the gravel and keep the nitrate, which are non-toxic, at a level so that your algae doesnít grow out of control.
This photo shows a twenty gallon tank with the logs siliconed in place and the rocks and gravel added.