My Lfs store uses these as sumps they have a bunch of them and all there tanks look clean. Its "the fish tank" in saddle brook if u are ever in jersey u can check them out in person and see how they are running them.
A lot of people use them as sump and also as frag tank too. I've seen people use them to cure LR or keep LR in and use it as a sump. People using it as a frag tank will use egg crates to support the frags. Some people will even plum a number of them together.