Many possibilities. Could be copepods, ostracods, scuds, possibly a large rotifer if you have good eyes. If you can get a closer look with a magnifying glass and see the body shape you may be able to identify them. Also note their swimming style, jerky, smooth, etc. Google the above and look at their body shapes. Lots of microfauna is generally a sign of a healthy tank.
If they are VERY tiny, like microscopic almost and dart around in quick short bursts, most likely you have a colony of freshwater copepods. Look that up online to see what they look like. If I look really close in my tanks I can make out their silhouette which looks exactly like the pictures online. They occur naturally in most freshwater and marine systems, usually they get into your tank by hitchhiking on live plants. Either way, they are completely harmless to your fish and actually if some of your fish are smart and good hunters they will make great live food for them. My peacock gudegon hunts and eats them all day although shes the only fish I've ever observed be intelligent enough and able to see them. They are a good sign, they indicate a healthy established ecosystem.