Copepods, water fleas, planaria, aquatic white worms, and many other tings will be found in our fish tanks form time to time. 98% of them are harmless.
Little round dots swimming in irradic patterns - copepods
Tiny looking worms that incha long the glass - planaria.
Little white worms you find in the substrate 1/2 inbch or so - aquatic white worms.
Little dot looking things that seem to swim in circles and you can kinda identify different part of the body - water fleas.