i agree with the above statements. i had a purple and a yellow in a 75 that i got when they were about 3 inches from snout to the base of the fin. they ate like pigs and grew like monsters. when i upped mt tank to a 135 they both went in and love how much space there was. i was pushing it with two in a 75, youve already got three, i really wouldnt add a fourth, as you might have some serious problems! JMO, though
good luck
jon