An easy to care for, yet predatory fish?

Well I'm looking for a fish that will fit in a 20-30 gallon tank (not set up yet). Something that eats live food and will be worth the maintenance of caring for the tank. I know about oscars, but I'm looking for something more like two-spot pike cichlids, a fish that's agile. And if you have any tips, those would really be appreciated as with my small 10 gallon tank I had a while ago, with small herbivorous fish, kept getting diseases for every fish I added to the tank! I also heard large tanks are more stable with water cycling and all, is that true?