Ya clownknifes get pretty big, had one that was 24" and around 2 yrs old. I kept it with a flowehorn, they got along great, the flowerhorn tried to pick on him a first, but he didn't take any crap and showed the flowehorn he was definatly tough. One recommedation though, make sure the tank is well covered. The one I had would always act very strange when it was going to rain( I know I sound crzy but its true) then one night after a big storm I got up to find my beloved knife dead on the floor. Very sad. And the tank had a cover on it, but it just wasn't heavy enough