1 or 2 shovel-nose catfish would provide your customers with something fun to watch, they spend quite a bit of time digging up the bottom so maybe only put plants near rocks or driftwood. They are fairly cheap when they're small but grow quite large. You could Also do a large number of Port Hoplo catfish. You can get them at around 2 inches long and they'll grow to 6 inches. Hoplos are nice because they really like to hang out with each other and explore everything you have in the tank. They are also compatible with any fish that are also passive. Customers will probably enjoy watching hoplos be swimming along merrily before stopping in mid flight and hang in the water for a few seconds, before taking off in a completely different direction. 10 would be a great addition to any big community tank.