You will have a lot of people give you a lot of different answers to this question. Heres my experience: I have raised Several strains Flowerhorn fry, Jack Dempsey Fry, and Synspilium Fry. I once raised about about 400 FH fry in about 90 gallons of water (as I had divided my 125 to give the fry most of the tank.) I raised them to about 2-3 inches so that I could not only see them grow up and color up but also so I could get the most money selling or trading them. The 120 gallon set up you have will probably be good for the next 4-6 months till they reach a size of 2-3 inches. Just keep them well fed (so they dont nip at each other as much) keep the water clean (do water changes of 25% once or twice a week) and as they start to get bigger, around 2 inches, keep an eye on how aggressive they get with each other. If you start seeing every fish in the tank having injuries like ripped fins then its time to start selling or trading them. Im looking forward to seeing the fry take over this tank. Keep the pictures coming