Mantis Shrimp - Powerful striking power can stun or kill a fish.
Coral Banded Shrimp - are advertised to clean fish, but are known to capture and eat them.
Ich - one of many diseases fish suffer from. Left untreated, the fish can die.
Lionfish - will eat anything that fits in their jaw.
Oxygen Deprivation - due to lack of circulation in a power outage.
Temperature issues - due to overheating or undue cold water can kill fish quickly.
Fighting - some fish will fight due to: same species, same shape, territorial issues... do your research before you buy.