What would be the biggest fish I could put in a tank with a 18"x20"x28" foot print (40pillar)? I don't seem to have luck keeping little fish so I was thinking one fish that would stay in the column. The only fish in there now is 2 ywg. I've lost cardinals, clown gobies, dart fish, ect. Thanks for the help in advance. (I'd do seahorses but I have a nem and other stinging corals)