nobody said 3-4 someone only said twice a week, but yes if they were a mated pair then this would be different as they would hold a territory together, with really good filtration, and keeping up with water changes, you could make a 75 work for a mated pair or 2 that werent fighting but i personally like for my fish to have a little more swimming space than that, iv seen videos on youtube with 2 oscars in 75g tanks and they just seem a little cramped to me, i mean im sure as long as you kept water clean they would grow fine, but i think they would really thrive better in a larger tank, i have a 125 and never had more than 2 Os in there at a time with other dither fish like tiger barbs, i like being able to see my fish have plenty of room to swim around and interact with others in the tank as well as me