I know the title promised rotifers, we'll get to them. But this was the first thing I spotted and I thought it was a rotifer - I was completely wrong - there's nothing wrong with being wrong - as long as you learn from it and I'm not a professional pond dipper so I have much learning to do.
Music - Aquarium by Kevin MacLeod
But what is it? It's a vorticella - they're single cells. They stick to things with their long stalk and contract any time they sense danger. The thing that looks like a disc spinning around is a crown of tiny hairs - cillia beating up and down creating water currents to drive food towards it.
Vorticella are important - they filter particles out of the water helping keep it clear, By eating tiny bits of food - dead bits of bacteria and rotting stuff that are too small for most things to bother with, they concentrate nutrients into a neat tasty vorticella shaped snack for larger things like crustaceans and small fish.
By Rkitko - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=5837680
They also - and we're not sure how - stop mosquito larvae growing in water. Vorticella make a glue - a biopolymer to stick their stalks to things, it does something to the larvae. It might block up their pores stopping them from sensing things, it might act a tiny bit like soap - changing the surface tension of the water so the larvae can't breathe at the surface. Either way they could be an important tool in controlling diseases like malaria and dengue.