To me a 75 gal is OK for 1 Paratilapia for a few years, although males get quite large under good conditions, but to keep more than 1 in that size tank is questionable. Males are very intolerant of other males, and non-receptive females, although other non-Paratilapia species are usually ignored. Your pH is certainly fine.
A 6 ft tank would be better if you want to keep more than 1.
Even in my 6ft tans though, the alpha males tend to kill females not ready, when he wants to be. The tank in the video below is 6ft, and held 1 male and 3 females for a while, but due to aggression, had to be moved to a 400 gal pool, after he started killing females.