Black background + black substrate = good to showcase colorful fish.
I've had very good luck with rainbowfish in a 90g tank, though I have read that they do prefer a 6ft tank. Tell me what colors you like, and I'll tell you which kind of rainbowfish shows them off the best! They prefer a hevily planted tank, with a large swimming space in the middle and in the top. Though, plants don't do so good with UG filters because their roots eventually tend to clog the UG system.
If you've never had a cichlid tank, and want fish with a bit more character, a 65-70g tank is a perfect place to give them a try. African mbuna are nice because most of them stay under 6 inches, and you are actually supposed to overstock the tank to cut down on aggression, meaning a more active tank with more character than your average community, though it does also mean more tank maintenance. But they are about the most colorful fish you can get short of saltwater. South/Central American cichlids are good too, but tend to be a bit bigger, meaning fewer per tank, and it's a bad idea to overstock with them. Again, if you're interested in cichlids, let me know colors or behaviors you're interested in, and I can make some recommendations. Most cichlids don't get along with plants, and most prefer a rocky aquascape and sand substrate, though sand also may not be so good with a UG filter, but they can do ok with gravel as well, they just don't act as naturally.
I like to keep at least one pleco in every one of my tanks, specifically ancistrus (bristlenose plecos) in tanks between 40-100g. They only get to around 6", and I have a soft spot for their whiskers. They're also much more social than most smaller plecos. I haven't seen my clown pleco in well over a month!
I'd stay away from clown plecos, rubberlip (bulldog) plecos, striped raphael catfish, khuli loaches, and other fish (usually bottom feeders) that tend to hide away most of their lives, because they contribute to bio-load but you rarely get the chance to enjoy them.
I'll give you any more ideas as I think of them!