About 99,9% of the tiny lifeforms living in freshwater are totally harmless to fish and most are 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 be seen with the naked eye, so the first sign of infection is often gray or red skin and increased amount of 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 are harder to see, different types of worms are the most common in the intestines of fish.
Some parasites do attack 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 infected fish.