Where did you hear aquatic plants have no nutritional value? I think aquatic plants do have nutritional value, some more than others. The nutritional value will also be affected to some extent by the nutrients in the water the plant has been growing in. This is the same as with regular vegetables that we eat. The vitamin/mineral content is affected by the soil they're grown in.
sQuirtle, if you buy something like anachris, it will come in bunches of strands. Don't put all of them in at one time. Give half of one, for example, and keep the rest in a bowl/tank of clean water. Rotate the plant with other foods. You can control how much your turtle eats in this way.
