Depends on the dimensions. When you're talking about big fish like common plecs/oscars/gibbis/clown loaches and the like, actual measurements are of more importance than volume.
It's generally considered cruel to keep a fish in a tank that is less than twice the fish's length in width, or they can't turn around easily; especially important for the less flexible fish, like plecs.
Most 75 gallon tanks are, AFAIK, 4' long, 18" from front to back and 2' in height; that's not really the right shape for a plec. A tank that was still 75 gallons, but 4' long, 2' from front to back and 18" in depth would be better, although still on the small side.
The minimum size tank I mentioned earlier (4'x2'x2') works out at 100g.
Personally, I wouldn't like to see a common in a 4'; they ought to have at least 5', IMO.