nice tank
Again you can keep monos in a 120 but in your case you already getting one that's 19" mine is 16". My male is thriving. I have 3 120's with peacock bass in it. If bought at a smaller size monos will grow as big as they can in a tank. with proper care and filtration monos will be fine. just because a certain fish may max out at a certain size doesn't mean it will even reach its max size. growth, colors etc all depend on the individual cichla.monos get 20 inches to 30 inches at max so that 4 foot would keep him alive but he wouldn't be thriving! im adopting a 19 inch mono in a couple of weeks so he surely would have a good time in a 6 footer.