The 40B will be a good tank for a mantis shrimp or an angler, either of these should be kept alone. You can have alot fun setting up a mantis tank. A wartskin angler if properly sized can be kept with a fu manchu or other appropriately sized scorps. The fu manchu is really the only lion that will not outgrow a 40B; if you keep any of the others long enough, they will need at at least a 55g, while I recommend a 75g. You can keep the yellow spot scorp and the leaf fish in trios, or pair them with a fu or other appropriate sized scorp. A dwarf Hawaiian eel or a chestnut eel(even though you'll never find one of these) is really the only eels that will be ok in a 40B. I keep a flame hawk that hunts live food in my small pred tank. You just have to size things appropriately so that one doesn't eat the other. Anglers will eat something their own size, while scorps and lions will eat slender fish easily and about 2/3rd their size. There is a dwarf octopus or cuttlefish, but this would be a bold endeavor. A lot of research on their specialized care, and hatching and raising cuttlefish is not cheap and is high maintenance,
You want to find out more about predator tanks and the care of predatory fish, click my name and "find all threads".
