Are they floating on the surface or are they down under in the water? There's lots of possibilities but they might be Daphnia, also known as Water Fleas. That's not to say they're a type flea, Daphnia are a crustacean, related to crabs. They're filter feeders so help keep the water clean. A photo would help to get a more positive identification. For that you might need a good macro lens.

Thank you for your reply. They appear to swim in random directions close to the surface, but some do go deeper.

Just refer to them as a 'type of water flea'. There's so many different types around. Most people don't even see them, and if they do they usually don't notice the details to know what they are. Do a search if you're interested in finding out more about them.