Strange organisms in my newt tank?

I am raising newts that are in the larvae stage right now. I expect some to die every once in a while. They can't all live. Today I saw an unusual amount of dead larvae in my tank! I got worried. Lately, these strange organisms have been appearing in the tank and I can't get rid of them just by doing a water change. They always reappear. They are tiny, the same size as the newt larvae, roundish with curved tails that propel them through the water. They are dark brown and there were LOTS of them today. Plz help I think these strange invaders are killing my newts!!!