Nyi,
When did you see these tiny white tadpole animals? At night?
From the description, they sound like copepods. They are good little animals to have. Their population will go up and down over the months. Don't worry about them. Although there are parasitic copepods, they are rare and larger.
The critters you see are not killing your fish.
In fact, it is likely your fish will eat them.
How about your water chemistry? Have you measured pH, Ammonia (NH4), Nitrites (NO2), Nitrates (NO3), specific gravity (SG), and the temperature?
Please post those results to help us diagnose your problems. Also post any other test results you run.
Thanks.