Cotton wool stuff on fish

[andy young]

I have recently lost some gold fish and a grass carp to cotton wool fungus. It's really horrid and grows all over the fish and rots there fins and fantails.
The koi vet has treated them with malachite I wondered
Has it been. A bad year for cotton wool stuff ?
Have any of you lost your fish to it ?
The koi don't seem to get it or the Chubb ?
Does it pass from fish to fish ?
What causes it ?
Is it linked to the u.v system or bio filters ?
Did they die because we did something wrong ?
Does it cause the fish pain ?

Thank you to anyone who knows the answers to one or several of these questions

Andy

 

[Meyer Jordan]

Saprolegnia is a fungal disease that is primarily caused by poor water quality usually combined with overcrowding of fish.

 

[Smaug]

As Meyer stated,poor water quality causes it.

 

[barryian]

Agreed! My fish are fine and been on top of water chemistry! All fish are growing since end of February and doing very well.