about 99,9% of the tiny lifeforms living in freshwater is totally harmless to fish and most is actually food for your fish.
in your comment you have found them to be cyclops and this is the ideal food for your fish both big and small.
here is a little about the dangerous types even if it unlikely you have any of them in your tank.
most of the external parasites are too small to see with the naked eye so the first sign of innfection is often gray or red skin and increased mucus on the skin.
the other part of external parasites is quite large and easy to see like fish lice and hookworms.
most of the internal parasites is harder to see,different types of worms are the most common in the intestins of fish.
some parsites do atack the gills of the fish,different types of gill flukes they are hard to see without a microscope but one can often see discoloration of the gills in innfected fish.