Should You Feed Live Feeder Fish?
Many owners of large tropical fish, such as Oscars, like the idea of being able to feed their fish live foods. A large fish can easily eat earthworms, frogs, small mice and yes, other fish, and whilst some people think that feeding one living thing to another living thing is barbaric, others see it as nothing more than the circle of life. Live food is a debatable subject for many reasons, this article will focus on feeding live fish to other fish.
First of all, if you are going to feed smaller fish to larger fish, it is a good idea to breed the smaller fish yourself. Feeder fish that you can buy in pet stores often carry parasites and diseases that can kill your prized larger fish. Hole in the Head disease (HITH), is a common affliction amongst large fish who are feed feeder fish. There is no official link between feeder fish and HITH, but there is plenty of anecdotal evidence to be found and most experienced keepers would rather feed their fish nothing at all than resort to feeder stock which often lead to outbreaks of Ich and other fungal problems.
The other thing one should be aware of before one goes gung ho into feeding live fish whilst singing 'Circle of Life' is that most feeder fish carry relatively little nutritional value. It's not 'better' for your fish to be eating other live fish. Fish foods from respected companies such as Hikari are formulated to be far more nutritious than a small feeder ever could be.
Feeder fish also contain an enzyme which breaks down thiamine, an important vitamin for fish. So, contrary to what might seem to be common sense, feeding your large carnivorous fish solely with feeder fish won't make it more naturally healthy, it will make it vitamin deficient, and more likely than not, sick.
By now it should be clear that feeding live fish to other fish is something people do for their own amusement, not for the fish's benefit. Some people like to watch their fish eat other fish, and though the drive is somewhat sadistic, there are plenty of people who do it. If you do insist on feeding live fish, I urge caution. Know the fish that you are feeding are clear of parasites and disease and only feed 'feeder' fish as part of a balanced diet that also includes a formulated commercial brand of fish food.
TL,DR: Feeder fish aren't really good for your carnivorous fish, can cause disease and have little to no nutritional value. Use feeder fish at your own risk.