White cotton threads in tank

[Meg8852]

Hi I just lost a catfish of 10 years and a betta trying to figure out what this is in the tank. Please help I want to identify the problem before adding new fish. Do I need to completely drain the tank? Or can I just treat?

Thanks!

 

[Goldfishexxpert]

It comes from the fish’s head or gills, happened to my goldfish, it means your fish is very sick or it’s worms, you need to get medicine for fish immediately