The guppy lifespan isn't as short as most people say. that's because they're usually considered "easier" fish and aren't given as much care; therefore, water quality declines, leading to quicker deaths. They're also used as feeders; so of course the lifespan might be shorter in comparison to say an arowana or oscar. My vote goes to convicts or HRPs. Breed easily, and don't die as "easily" as guppies or other livebearers. Otherwise maybe a pair of kribensis would be nice, but some have quite the attitude.