in the predatory sections theres a fair bit about them, they vary in price and availabilty. Most decent shops should be able to get them for you. Wildwoods has someonline, BAS has some. Or if you see anything you really want you can ask on here see if someone has any.
I've got Channa Orientalis and theyre great, grow to about 4-5 inch , they live happily in a 40gal but tbh could live in smaller accomadation. They're easy to look after fish , if in a species tank do the water change monthly , don;t need high light just some good filteration and theyre happy. I can't remeber when i last did my water change on mine be a wile now .
Theres things like puffer fish as well which can probs go in there. most are brackish i think but some are full tropical.
Best thing to do is browse the net , see what you like then come back here and we'll advise on it.