Those almost sound like mosquito larvae, but they shouldn't be in non-stagnant water. Mosquito larvae have spiny heads also, so they would be easy to spot. (I used to go to some stagnant water near here and collect hundreds of mosquito larva for my cardinal tetras -- they loved them.)

Anyway, I had a similar problem with larvae of some sort in my shrimp tank in the past. I put a little fish or two in there (tetra or whatever) -- the larva will be gone in a day. It's been over a month since I removed the fish and the larvae have not returned yet. Of course, if you have newly hatched shrimp, you will lose some or all of them to the fish too.