Well, it's actually a 12gal.  I'm hoping to get a 30 so he can have a partner and more plants.  As far as the red tails go, I did have three as recommended, but one had died.  He was a little lean when we got him and I was hoping he'd grow out okay.  I was wrong.  These two have been cruising together and no aggression towards each other.  I'm still lookin' at getting a 3rd though.