Well, in my experience, it has not done them any harm. As long as you keep up on weekly water changes, and you feed them , they should be fine. Also, Convicts max size is 6 inches. Now, my 29 is about 16 inches long ( i think, not posostive, to lazy to go check), and around 8 inches wide. So, if and when they reach full size, the fish still has a extra 10 inches of room length wise, and 2 inches in width , allowing them enough room to comfortably turn around/do what ever they please. My 20 gallon falt hex they are in ( atm) is perfect for them, since it has such odd dimensions.