Keeping that Dovii in a 75 as a grow out tank is just fine just as long as its not, of course, going to be perminate. This thread needed cleaning up. It got really childish and pathetic (but I kept reading...lol.) It was like 4 whole pages of "I'm right you're wrong...and you're an ***hole"...in a nutshell.
Water changes are something you cannot get lazy on. I'm going to go out on a limb and say once that fish gets over 10'', water changes should be done at 40% every third day...I would just start with that routine now instead of waiting.
I can't remember when you said you were getting your larger tank (think you said 200 gal) but when kept in good water and fed healthy you're looking at .5'' to 1.25'' inches per month slowing down at anywhere from 5''-7'' inches if its a female and 7''-9'' if its a male (if I remember correctly, but to be honest I'm not sure). If and when he reaches 15.5" (which he might not in a 75) get him out asap.
I don't know what you have for decorations in the grow out tank but lose everything except one or two plastic plants or rocks in the corners on opposite sides of the tank. Give as much swimming space as possible. What kind of filtration are you running on the 75? Too much is not enough. Get a couple of maxi-jets 1200's in there and get him moving around. Thats all I can think of right now and I feel guilty that I'm up here on the puter in stead of downstairs with my family on christmas day.
Merry christmas fish people!