my friend has 12 in a 120 gallon,then tranfered it in a 240 gallon,now it was reduced to 10,I think it will stay 10 there for life,as long as they have established thier own territorry and is accustomed with each other...for a 75? i would say 4....well if wanna try your luck try starting from 6pcs..its really a case to case basis