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Just today, i noticed several hundred, probably thousands very very small micro-organisms swimming in my RES tank.
I understand there are benificial bacteria, but i did not think those were visible to the naked eye. Are these good for my
turtle? I am doing partial h2o changes once a week. His behavior has not changed, but i am more concerned about how
whatever these are, that they might present a health risk to my home. Any info would be helpful.

Most likely it is an infestation of fly larvae. A thorough cleaning and replacement of filter media should help keep it under control. Unless they are parasites, they don't usually present a problem but I don't think you want to keep them around either.