They are mites feeding on the biofilm on the top of the water. Several fish foods contain fish oil, which collects on the top of the water over time, and the mites show up to eat the microorganisms that are feeding on the oil.
The mites are harmless, and good fish food if one of your fishes nabs one. But, if you want to remove the oil run clean pieces of paper over the top of the water. Depending on the size of your tank, it can take several pieces of paper to remove all the oil. You could also switch to another brand of fish food that doesn't contain fish oil.
Welcome to the forum by the way.
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