If they swim in the water column like a snake, head going in one direction, body wriggling, detritus.
If they swim in the water column like they can't control where they are going, nematodes.
My vote is some form of detritus. Not planaria, nor rhabdocoelas.
And yes to the copepods! Saw one definite female! A lot of people think their males with two sacks.... those are egg sacks! Not the other kind! lol