I currently have a cycled 20G with an Aquaclear 50 filter. In my tank I have a Fire Shrimp three Emerald crabs and a twin sport goby. I would like to start adding fish but I'm more interested in invertebrates. I really would like a tuxedo urchin but am worried I don't have enough room. Any suggestions? What will go well with my other fish? I had a pseudo but he attacked everything and went back to the LFS. Thanks!