How about a single male convict?Trust me ,they're pretty much a mini red devil or midas.Sure it does take time for them to grow out but it's totally worth it.You could keep a few or a pair in that tank but I wouldn't recommend it because they're probably gonna pair up and overwhelm you with fry.You could add some fast moving dithers to make the tank look more lively.Not into cichlids? Say no more .You could also keep the some types of upper jaw polypterus,sure a 75 would be ideal but most people say the minimum footprint is 36x18 inches.I'm no expert on bichirs but i've been told that it's floor space>height for bichirs.This guide will help you out https://www.monsterfishkeepers.com/forums/threads/everything-you-need-to-know-about-bichirs.729562/.