If you want to go with predatory fish, that's great. But we need to be clear here on whether you're looking for something that'll take commercial foods like pellets, or whether you're prepared to offer suitable fresh, frozen or live foods. It's crucial to understand that the use of goldfish, minnows and other feeders just isn't acceptable. Even if you don't think the use of live fish as food is cruel, feeder fish bought from pet stores are incredibly unsafe, an NO expert fishkeeper recommends them. If you choose a species of predatory fish that only takes live fish -- and the only ones I can think of are
Chaca spp. catfish and South American leaffish -- you'll need to produce those live feeder fish at home. Typically these are juvenile livebearers, killifish or cichlids (
Chaca spp. only take bottom-swimming fish, so cichlid fry are usually the only option).
Pretty much everything else can be weaned onto either live invertebrates such as worms and river shrimps, or suitable wet-frozen alternatives. The predatory fish I recommend as a good species for casual aquarists to keep is the South American freshwater barracuda, actually a characin,
Ctenolucius hujeta. This species is adapted to poor-oxygen environments, and that gives it exceptional tolerance for the conditions found in most aquaria. It is also very adaptable, readily taking wet-frozen foods like tilapia fillet if offered using forceps. Earthworms are also enjoyed, and an excellent source of vitamins and minerals thanks to their herbivorous diet and mud-filled guts. They are peaceful, and get along well with each other as well as fish too large to swallow whole. Best of all,
Ctenolucius hujeta is very attractive but not too big, and 2-3 specimens can easily be held in a 210 litre/55 gallon tank.
I'm making these points here because too many beginners to the hobby associated predatory fish with the mindless use of live feeder fish. Many predatory fish will live more than 10 years in captivity, and if you choose a species that only takes live food, that quickly becomes very expensive. So it's a good idea to start off thinking about what foods can afford to provide, how safe those foods will be, and then choose a predatory fish that matches your resources and budget.
Cheers, Neale