You could keep a shortbody flowerhorn in a 55 by itself, they max at 8 inches, well, I've never seen one bigger. You might get away with a parrot for a tankmate but the parrot will have to be bigger than the flowerhorn from the start and even then it's a chance. I have a shortbody male flowerhorn and a male parrot together - it worked, but only because the parrot wouldn't take crap form the flowerhorn and it was bigger. The flowerhorn wouldn't show good color because of this, he faded out in submission, so although it could work, you wouldn't get the best out of the flowerhorn probably.