I always though 18 inch was plenty until I had to temporarily transfer my 11 inch Piraya from a 24 inch wide tank into a 18 inch wide tank. What a difference. Seemed like he had no room to properly turn around. You can keep probably do 3-4 Caribe in a 72*24 tank. I know somebody who has 3 in a 220 and it looks perfect. They are 13 inches long.