Copepods is my guess and it is actually a sign of a healthy tank. I have a ton of them in my tanks but they are a microscopic variety. My fish chow on them when I take my filter sponges out as they flow into the tank.
I have seen a site where they actually grow a certain species specifically to feed fish but I will have to find it again. Can't find it yet but you will know if you find it, it looks like a big ugly tadpole!
Well couldn't find the site but did find some information.
Copepods are a huge family of many speices all of which are predatory. They have small fin like structures almost looking like gills and have a "foot" that is used to grab their food out of the water column. Only predatory to animals 1/4 their size until they get larger than a pea then can eat much larger creatures. They are common in all qater bodies and are found in well maintained aquariums and are and indicator of water quaility although they are also an indicator of excess food which is feeding their food source.
When I found mine, I watched them eat the nematodes in the same sample, kinda cool but kinda makes you wonder what other "things" are in your tanks.