what do they survive on?
Small fish, fish larvae, squid, and crustaceans, etc.
Ocean sunfish
However, genetic analysis reveals that sunfish are actually generalist predators that consume largely small fish, fish larvae, squid, and crustaceans, with jellyfish and salps making up only around 15% of the diet. Occasionally they will ingest eel grass. 
what is ocean sunfish? One of the heaviest known bony fishes in the world.
wiki:12087786
Ocean sunfish [SEP] Feeding
The diet of the ocean sunfish was formerly thought to consist primarily of various jellyfish. However, genetic analysis reveals that sunfish are actually generalist predators that consume largely small fish, fish larvae, squid, and crustaceans, with jellyfish and salps making up only around 15% of the diet. Occasionally they will ingest eel grass. This range of food items indicates that the sunfish feeds at many levels, from the surface to deep water, and occasionally down to the seafloor in some areas.