what a great deal! since 40 hex does not as much floor space as equivalent rectangular tanks, the potential livestock should not be actively swimming species. if you want to try something different, you can set up a species tank. there are certain eels like banana or viper moray that do not exceed 12". i think this would be an interesting set-up. another option is a seahorse/pipefish tank. since your tank is hex, it'll be perfect for seahorses, which demand some vertical space. of course, this won't work if you're not willing provide live food for them....there are many options...like anglers...etc...you can also set up a peaceful community tank as well....