Hey guys.

Recently i purchased 3 new congo tetras for my tank to add to the school of them. After i few days i noticed one has a small white spot on its fin, so i instantly started doing regular water changes. Then i woke up yesterday and two more of the congoes have developed fungus growth, one of them being really serious and in its mouth.
Iv had this tank for two years and the only fish that have had fungus are the occasional feeder fish, which i scoop out immediately.
I've also had a large protein build up in my tank, and so only a week ago introduced a bag or "Purigen" into my eheim canister, could this be what is stressing the fish out?
Anyways, any tips guys for combating this?

Thanks