As it turns out I had some molly fry in there for some time with some ghost shrimp. The mollies were just in there until I could find them a new home. Unfortunately in order to get them all out I had to disassemble the whole thing. Sniff. I was then using it as a wild plant quarantine tank for awhile and now it's got nothing but dirty sand in it until I can find some time to give it a makeover.
I found the rock formations to be a double edge sword. It was cool to see the fish and shrimp swimming around in there but there's the problem of getting dead shrimp out as well as dead plant material. The rocks looked cool for a long time but then a dark almost black algae formed all over the tops of the rocks and looked bad. I don't like the idea of having to disassemble and redo the rocks with the fish in there so I don't think I'll be doing rock formations like that again.