It has been a long time since I had fighters and I wasnt that good at it, but
The more water you have the more room the fish have so the bigger they can get BUT you also have to be carefull because the food can get _lost_ in the extra room
Also the more you change the water the harder it is to keep stable so the more work you have to put in to it to maintain parameters
BUT the more water that is changed and the more food that goes in the fast fish grow
having more volume early in the piece may not be the answer but having aged water good to go a few times a day might be
Maybe seeing its hols and hot a water change 3x a day and feeding straight after might be a good comprimise
only needs to be a small change each time
say 20% or so...trial and earror untill you sort what work for you
My discus fry used to get 200% water changes every day...but it was warm aged water and they were feed 6 times a day
Maybe?
LOL
HTH