Are you planning on keeping the fish till it dies of old age? I am assuming you are going to get a male. I would not like keeping a full grown male in a 75. It does not mean it can not live in it for life, but to me I don't think its ideal size.
If you plan to move on to the next fish after keeping jags for a while, I would go for it in a 75
My male is 14" at 2 1/2 years old I keep him and a female alone in a 110 and theres more then enough room, I previously had them in a 90 till they touched a foot in length. Best of luck
A well cared for male jag has the potential of reaching twenty inches in length. For me, a 75g is too small for a lone male. Ideally, you'd want a tank with at least a six foot by two foot footprint.