I wanted to add some additional information regarding the nature of the stuff that the turtles are discharging into the aquarium. Most animals have a particular chemical that they excrete which represents the breakdown of proteins. For humans it is urea, for fish, ammonia and for birds and reptiles (such as turtles), it is uric acid. Actually, turtles excrete both uric acid and urea...the white material you refer to is probably uric acid. First, I don't believe that activated carbon will bind and remove uric acid. However, if left alone, uric acid will be converted to ammonia (same thing that fish excrete) by common bacteria and will then enter the aquarium nitrogen cycle. The probem is the effect of this acid upon the chemistry (pH)of the aquarium water and in particular, the effect upon the delicate membranes of the fishs' gills. Uric acid is a weak acid but can be quite caustic in concentrated form....certainly sufficient to burn through the clear coat of an automobile finish if not removed. I would keep an eye on this and perform more frequent water changes and aquarium maintenance.