fish food. could be daphnea, cyclopse. do they have two sack like things?
I have seen that damage of the bigger oval ones. I have them in my macro tank...I am tempted to take her out for a few days and release an army of zebra danio into it. I can't keep any fish in there with my macro because she'd eat them.The little round ostracods do little harm if they are not very numerous, but the bigger oval ones can do real damage by eating away at the edges of plant leaves. The bigger ones look like this (first picture):
When they are damaging my plants, to me, they look like this (second picture): They are very hard to get rid of because they lay eggs that are resistant to drying and chemical treatments.