I totally agree, "there is no suck thing as too much water changes". The more the better, only if your fish is used to huge waterchanges.
I do 85% to 90% weekly waterchanges on all my tanks w/ water conditioner of course.
I believe your fish rammed the tank so hard and broke its spine or something. Its common for large CA fish ramming tanks with full force. Hey, look at the bright side. You can get a male jag now with much, much better patterns and attitude. If you are in Vegas. Ill have some 4" males I could give you.