i have a 5 gal tank, w/ lr, no sand, some sort of mushroom, some pulsing xenia, a brittle star, 3 hermits, and all the tiniest little snails that hitched in, and one oscellaris clown.
my fish is happy, he eats well, and he swims around checking for stuff on the rocks.
most importantly, i haven't had any problems so far, since august (i know it hasn't been too long, but so far all is well). my water parameters have been perfect.
it is kinda hard to care for. it will overheat quite easily, so i have to leave the lid open most of the time, since they keep the heat on in the dorms all the time in the winter, so the water evaporates quite quickly. i have to top it off at least once a day. it takes a lot of care. if she's immature, it might not be such a great idea.