Solid comments above. Tank size is certainly relative. That said, not only can you do a good bit with a 75 gal tank with good filtration and sufficient water changes, if you keep it moderately stocked and don't quickly use up its realistic capacity with just one or two fish, but it tends to be more rewarding in terms of allowing more variety, closer to natural behavior, and fewer health and water problems. An alternative could be to keep a single of a super nice, wish list fish that fits the tank but, being territorial or aggressive, is typically better off alone or with a few appropriate dithers, (except in much larger tanks). Something smaller than an oscar is better, or you end up with the common cliche of a small cage of a tank for a large fish that deserves better.
That said, more filtration would help and is not a bad idea, but you're still limited both by tank size and your max water changes.