Copepods are small little critters that like look insect larva. Found often in marine aquariums as food for corals. They live in freshwater as well, and are eaten by fish.
I was actually mistaken, and the things in my tank are called Amphipods. Here's a ugly shot of them from Darrell Stephens when he had this infestation. You can see them swarming the cucumber slices.
These Amphipods descimated my tank. They chewed the leaves of my jungle vals, and Java Fern to a pulp. Literaly nothing left but roots and rhizomes. When in a shrimp only tank they are nearly impossible to get rid of. But when you have fish, they'll quickly get eaten.
-John N.