what’s your temp look like in the tank? Increase it to 88/89 degrees add salt make sure you have plenty of aeration feed 3-4 times a day maybe more if they will accept more, increase water change intervals to every other day minimum 50%. I have grown most of my bass if not all from 1.5-2.5” I have never looked or wanted to purchase bass a 5” or more, they are much easier to pellet train at smaller sizes. If you need to add a partition in the tank to get smaller bass to bulk up to the same size also any dithers in the tank? Maybe add some small dithers in there to help with competition for food and spread out aggression (swordtails, Mollies, they will be eaten when bass get some size but they serve a purpose). If you add partition to separate smaller bass make sure to add dithers in with him. Good luck
edit: most of my bass if not all started in a 10 gallon then to 40-75-180 then to 600 gallon