First off there have been a lot of stupid answers on this thread.
Your tank is a bit slim, I've got some 50ish gal that are 19.5in front to back, however there are still a few to go at:
pim ornatus
pim pictus
pim blochi
95 percent of synodontis sp.
hoplo
flag tail diadema
marble bagrid
jaguar cat
all of these fish will thrive on a flesh based diet whether this be small fish or insect larvae and most can be kept in groups of their kind.
if you look any of these up on planet catfish there is a facility to search for similar or same sized fish.
you are doing the right thing by checking before stocking-dont be put off by some of the previous answers.