Most fish CAN be, it's just not practical to keep arapaima/alligator gar/mekong giant catfish/other 4+ foot long fish in aquariums (I'm talking muskie/pike), especially when they eat live food because live food can get expensive and when feeding live, you're usually bound to get your fish sick unless you raise your own feeders (which is hard to do on the scale that muskie/pike eat). Pickerel, can be kept in the home aquarium (albeit it's a bit of a stretch, but still possible), but I imagine it would be very hard to pellet train it.
I can't help but feel that I'm being interpreted as someone who actually endorses keeping monsters like that; nothing could be further from the truth. I avoided joining MFK for years, simply because I didn't approve of the endless threads asking if a Redtail Cat really needed 100 gallons, or if it could be kept in 75...or inquiring about the "minimum" tank size required for fish species ABC or XYZ...or stating that the poster had just purchased some other potential giant, knew nothing about it, and was now asking for advice. I outgrew my fascination with true monsters about 30 years ago, going through the phase as so many others do; there are plenty of members here who are still deep in that mindset, but my interests lie with much smaller game now.
I mentioned Grass Pickerel as an alternative that can be responsibly kept in large tanks, hoping perhaps to illustrate that the interesting Esox genus contains some fish actually suited for home captivity. I also alluded to the idea that these fish do not require live food. Actually, very few fish do; the vast majority of fish species will quickly and easily learn to accept dead and/or prepared foods, with some effort on the part of the keeper. Esox are no exception, and as I mentioned don't even require "training"; they will feed on dead food in nature, and quickly transition to prepared foods in captivity. Pellets for carnivores weren't available back when I had these creatures, but homemade gelatin foods made with ground whole fish/flakes/Trout Chow were easily produced and were readily accepted. I don't
imagine that; I know it. I would hazard a guess that something like 95% of carnivorous fish being maintained on live feeders are being kept that way because of the entertainment value afforded to their owners. I don't do it, there is no good reason to do it and many good reasons not to.
So, to be clear: I don't say you should have a Muskie in your living room, and I am still a little shocked at the number of keepers trading in species that, by comparison, make that a reasonable plan. I'll just add this thread to the list of those others that I avoid reading, and will bow out now. Cheers!