You can put a Spotlight above the water , you will attract Zooplankton and you just have to pick it up with a bucket just be careful that there are no predatory insects but in my pond with the fish I have never seen a predatory insects.
I put a picture as an axample, on the picture you can see small Daphnia attracted by light.
Depending on the season you will attract different critters.
There are so many that for my needs I put a small pump under the spot.https://ibb.co/VwW7FMh
But Daphnia is for the fry, for your larvae it takes very small Zooplankton like Mr Cody said.
Or you can make infusoria, take a one liter jar with pond water, for the bacteria you can use bread, hay or milk if not pasteurized, in Europe they use milk to raise different Zooplankton, because it is not pasteurized.
In canada all the milk is pasteurized, I don't know in the U.S.A