Jellyfish are opportunistic carnivores that will eat almost anything they come across. Smaller jellyfish consume plankton, larger ones eat fish eggs, crustaceans and snails, and the very largest jellyfish can catch and eat whole fish. Jellyfish can also be cannibalistic.

Jellyfish kept as pets require a steady diet of brine shrimp to survive. Baby jellyfish need very small freshly hatched brine shrimp. By adulthood, they require fully grown brine shrimp. Feeding adult jellyfish kept as pets is easier because they can survive off frozen food. Two common foods for adult pet jellyfish include shaved mysis shrimp and cyclopeeze — both of which are available at pet stores.

When feeding a pet jellyfish, the brine shrimp or similar only need to be placed in the tank with the jellyfish. The jellyfish will sting the prey and draw it into its own mouth.

Jellyfish are a type of plankton, meaning they are not strong swimmers and at the mercy of currents. When two currents meet, large concentrations of jellyfish, known as blooms, can form. When jellyfish blooms form, they can eat almost everything in the water. This can cause problems for commercial fisheries because the jellyfish will leaving nothing for the other fish in the same size category to eat.