Discus looks mouldy and getting worse

[Bengl]

So I have 5 discus and I hand feed them. When I did this on Monday I noticed one of them looked like it was covered in salt and looking at it, it looked like Ich.

So I bought some Waterlife Protozin the next day from my LFS and also did a big water change.

I had to remove some parts of my filter (carbon etc...) and when I turned it back on it spewed brown all over my tank which I am trying to clean but also trying to avoid replacing too much water at once but it is getting cleaned up.

I have been checking the water (amm. ni, no) and it's always been fine (except for yesterday when it had very high no).

The infected discus has since gone really, really white and looks like it's now covered in ich and fungus whilst also suffering fin rot. None of my other discus have anything except for one with a very small amount of ich-looking symptoms which have been there since Monday and haven't got worse or better. My rummy-nose tetras also have ich symptoms and they keep dying (5 dead in 4 days).

I have attached a photo I've just taken of my discus and it looks in a terrible state.

I don't want to add salt as they also share the tank with an ancistrus.

Oh, all my other discus seem very lethargic and uninterested in swimming around but show no other symptoms. And when the severely infected discus does decide to go for a swim it swims fine but only for a few seconds.