Does anyone have a decent photo of these 'water fleas' without magnification?

We've got some critters in our grow tubes and I'm trying to determine if they are beneficial or not. These things are TINY....like smaller than a mustard seed tiny, but they swim around rapidly. I've got pretty darn good eyes, and even *I* can't get a good look at them they are so tiny. I suspect they were imported with the 'ditch fish' (minnows) I brought up from the canal behind the house to feed the brim.

I understand that the critters being discussed are beneficial, which would be a good thing as we've got scads of them in the tubes. that being said...are there any tiny critters one would want to look out for that would be detrimental to system health? (so far my google-fu has failed to provide any enlightenment.)