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I have recently performed a water change on my tank, I have the lights on so I can see what I'm doing, and noticed hundreds or thousands of tiny white worms floating in the water after I had refilled the tank.

I've attached some pictures which won't be much good, but they are tiny maybe 1-2mm long and as thin as human hair.

I have been feeding this tank fairly heavily as I recently got the fish and a couple were thin, although the tank has been setup for almost 2 years and has had a and around 40 black neon tetras in it for that length of time.

Other than heavy feeding I did treat with flubendazole as I think the fish may have had some internal parasites, could these be newly hatched parasites, which were safe from the flubendazole as they were in an egg case during treatment? Or have just disturbed some small organisms in the sand which have only appeared to eat leftover food? They are actively moving but don't seem to have much directional control!

Thanks for your help.

They are free living nematodes. I see them in my shrimp tanks regularly.

Small fish will eat them often. But shrimp and larger fish seem to ignore them. They are harmless.

Vacuuming the substrate during water changes will help remove them and their food supply which will reduce the population to unnoticeable levels.

Andy

Thanks Andy that's good to hear, hopefully the tetras will eat a few of them, and I'll have to thoroughly vacuum the sand a few times to see if it reduces the population. I'll also reduce the feeding slightly now the fish are settling in.