Most Cats are great preds will eat and gobble up anything that fits it's mouth. RTC are great but ZZ's are better. LMB is another plus. Some other ones are Golden Dorado, SH, Oscarfish(still one of my favs), african aro, and...so many more. But above all if you want an aggressive pred that's beautiful. I'd say a Jar is #1 on the list. It's doesn't get enormous like most cats and other preds that get huge. Most other fishes are aggressive but not aggressive eaters like the jar. If not jar then LMB's, PB's, or Perches they are aggressive eaters that don't get super huge. Or you can always go wil the smaller fishes that can be kept in a 75 gal. Grass pike, warmouth, bichir, smaller cats like bullheads, gulper, driftwood cat, so many possibilities. However, I'd go with Jardini personally as far as best active loner fish and a ZZ for best non-active loner fish.