http://www.bestfish.com/ug.html
Read this article, it is not necessarily aimed towards filtering for a turtle aquarium, its more towards fish, but its the same idea, and gives you a pretty good idea of under gravel vs power style filters.
Read this article, it is not necessarily aimed towards filtering for a turtle aquarium, its more towards fish, but its the same idea, and gives you a pretty good idea of under gravel vs power style filters.