I had both planaria (flatworms) and nematodes (roundworms) they were both small and you could really only see them squirming around in the substrate pressed up against the glass. I think they probably hitched a ride over on the substrate or plants. At the time I had no fish and was overfeeding a bit which resulted in a fair amount of worms (and also pond snails). I cut back on feeding and there are almost none left. Hopefully the fish I just added will snack on the rest of them, although I don't think they're really hurting anything in there.