Some fish are powerful weapons used to destroy enemies, or yourself. Sharks and piranhas are carnivores and they are eager to eat everything soft and pink, including people. Some fish, like the legendary fugu, are safe to eat if properly cooked, but may be extremely poisonous. Some fish carry poisonous spikes, and if a person steps on such fish he gets hurt, and sometimes may even die. In addition, some useful substances can be extracted from fish.
Impotence curing medicine produced of sharks and cod liver oil are probably the best known.
Useless fish Useless fish are none of the described above. Most of them are useless, because it is impossible to catch and make any use of them. Some of them are useless because they are very small. Dead fish washed ashore are mostly useless. Fish eaten by another fish instantly becomes useless.