probably aquatic nematodes or maybe annelids. i have those.
dont worry, they were in your bettas tank from before he hatched, so they are harmless. when he was smaller they were food, now hes grown spoiled on bloodworms and brine shrimp no doubt. but if one gets peeled off of what its crawling on and squiggles past his face he will probably chomp down.
i had planaria too. also food. and cyclops, and these little bugs on the surface, and a different kind of tiny worm. just took some moss from my shrimp tank and left it in a jar on the window occasionally adding a bit of fish food. then when there were little things everywhere (right around the time of the spawn) i emptied the jar in the spawning tank to have microfauna for the betta fry to graze on between feedings.
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