They're harmless but may irritate your fish if it got into its gills and etc. Those worms are a result of over feeding or bad water quality. Your filter is where majority of them will be hiding in. My best advice is to relocate the fish, drain the tank, clean the filter, let it sit in the sun for at least 2 days before setting it up again.
P.S. Clean water & constant water changes does not equal to good quality water.
P.S. Clean water & constant water changes does not equal to good quality water.





