Feeder fish
Feeder fish is the generic name for certain types of inexpensive fish commonly fed as live prey to captive animals, such as sharks, turtles and some other predatory fishes that inhabit fresh, brackish and salt water, such as oscars, gar, and rays.

Advantages of using feeder fish 
The species of fish usually sold as feeder fish are invariably some of the easiest fish for fishkeepers to rear and breed, such as common goldfish and guppies. Typically, these species are tolerant of overcrowding and have a high fecundity and rapid growth rate. This makes it easy for fish farmers, retailers, and hobbyists to maintain large populations of these fish that can be sold at a much more affordable price than the more delicate ornamental fish that require better conditions.

In some cases, species of predatory animals, typically large fish such as catfish and cichlids, but sometimes also animals such as freshwater turtles, are provided with feeder fish, because they accept them more readily than alternatives. Other animal keepers believe that feeder fish are particularly nutritious, being what their pets would eat in the wild. Still others view feeder fish as a stimulating treat that encourages predatory animals to exhibit their natural hunting behaviours. Some animal keepers enjoy watching the hunting and eating techniques involved when one animal eats another.

Disadvantages of using feeder fish 
Some aquarists view the use of feeder fish as cruel and unnecessary, arguing that once the feeder             fish is introduced into a small tank, it has no chance to escape, and such a contrived situation cannot be considered "natural" in any meaningful sense. Most predatory species that eat live fish can also be weaned onto dead alternatives. Some of the species used as feeder fish (goldfish and rosy red minnows) contain high quantities of thiaminase, an enzyme that destroys thiamine (vitamin B1) and when fed in large quantities causes nutritional imbalances. When bred and held in an overcrowded or otherwise sub-optimal environment, they may also carry bacterial infections and parasites, which can be passed along to fish that eat them. A major disadvantage of using feeder fish, particularly goldfish, is that they do not actually simulate what tropical fish eat in the wild.

Species use
Several fast-growing and hardy species are commonly sold and used as feeder fish. Depending on the locality, feeder fish may include:　
 Low-quality, common livebearers, usually guppies, mosquito fish and platies
 Small cyprinids, particularly rosy red minnows and goldfish
 Unwanted livebearer and cichlid fry
 Female Siamese fighting fish
 Young tilapia
 Defective and weak fry
 Young bluegill

Opinions within the hobby 
Although the use of feeder fish is fairly common in the United States, in the United Kingdom it is much less common, with aquarists and hobby magazines in Britain generally rejecting the use of feeder fish as being unnecessary and likely to cause health problems.