How small are daphnia?

[Herpin Man]

I discovered, by accident, that I have been culturing some very tiny critters in an indoor turtle tub. I suppose they came in on aquatic plants.
I don't know what they are. They are tiny black specks that swim. Even under a magnifying glass, they are just little black specks. The newt larva seem to like them.
I've never used daphnia before, so I don't really know how big they are, or what they look like.
Any guesses what they might be?

 

[Bellabelloo]

Have a look here and see if you can find them

Caudata Culture Articles - Aquarium Invaders: Photographs

 

[Herpin Man]

I can't tell without a microscope. Maybe they're water mites.