I don't clean my glass often, and I can seen them crawling over the glass, maybee 4-10 per square inch. So lots.
There are different types, I get little cricket things, small (no bigger than 1mm), these are the ones the mandarin eat, then the larger cricket things (that are curled up and look like fleas), most of the fish eat them, often you see dead shells floating around the tank, they must shead their skin. Then I get the mysidd shrimp, they like lower flow, the dark, and can swarm in clouds some nights.
Pie