get the tigrinus they grow to 28inch max they dont swim around much and the best part is they are the best looking cat on the planet
in a 6x2x2 tank you will be luck if it grows much past 20inch
if you buy a 7inch one it will take 2 years to get to 20inch they grow fast up to 16inch then slow right down
mine is about 5 years old and only 24inch the trick is to get some large mid water fish like tinfoils or silver dollars as they will grab most of the food tigs are rubbish hunters and wouldnt catch much in the wild