Hi and welcome to the forum
Without a picture of the small white things I can't identify them with certainty.
Copepods are small crustaceans that live in fresh, brackish water and sea water. They are eaten by most fish and generally do not cause any problems. Although some literature states that male copepods can become gill parasites in fish. I have never had that issue but others might have.
The most common copepod in fresh water aquariums is called a cyclops. They are about 1-2mm long, white and they look like a Y. The two top bits of the Y are actually eggs that the females carry around, the males you can't normally see without a magnifying glass or microscope. Cyclops tend to flick about the tank, and you often see them on the glass where they move rapidly in a jerky motion, flicking from place to place.
They get introduced to aquariums on plants or in water from outdoors ponds. Most small fish will eat them.
If you don't want them in the tank you can try Praziquantel. This is normally used to treat tapeworm in dogs & cats but also works on fish and kills some small aquatic organisms. Or you can use Malachite Green, which should kill them but is pretty toxic. And if you don't have snails or shrimp in the tank you can use Copper Sulphate.
Personally I would nor worry about them unless your fish get sick.
