The collective noun plague is used for a plague of locusts or a plague of accidents.
A fluther or a smuth of jellyfish.
The collective noun swarm is used when mentioning a group of insects such as swarm of bees, swarm of loccasts, swarm of flies, etc.
The collective noun is a troubling of goldfish.
The collective noun'school' would be the best choice, for example: A school of small fish swam by.