Discus: Lump below dorsal fin - burst with white yellow stuff

Hi :) sorry for the long post; i figure its better to include anything that might be relevant.

I need help identifying whats causing a lump on one of my discus under the front of the dorsal fin.

The tank-
Its a 500L tank, with 5 discus, a bunch of neons and other tetras and a pleco. Its got a big Fluval cannister filter, is generally 27 degrees C, and has been cycled and running since the beginning of February.

I do 1/3 water changes every 5 days (once a week minimum). Since we are on rainwater I use that, instead of having to treat tap water every time. As far as I can tell this discus was 2nd from the bottom in their little group, and though being sick has allowed the last discus to steal his spot now, none of the others have been picking on him.

pH: 6.8 (our rainwater is usually 7.2-7.4 I only started adding buffer to bring it down last week)
Ammonia: 0-0.25 (I'd say 0 but the test yellow has the smallest touch of a green tinge)
Nitrite: 0
Nitrate: 5.0 (it generally falls between 0 and 10 pretty consistently)

My blue diamond ate himself silly and I added epsom salts 4 water changes ago (that fish is fine now) which are still probably still present in the water.

Since it is a heavily planted tank with a good light I have an ongoing algae issue, I usually leave the carbon in the filter and just do 1/4 doses of algae killer every now and then to keep it down, but last week it got out of control and I had the carbon out and did 2 rounds of a 1/2 dose of that; Its very well aerated tank and that didn't change the water readings though.

The fish-
I haven't seen him eat since last Friday the 17th.
Hes swimming fine, but has mostly been hanging back by the heater behind a bunch of grass.
The lump started out looking like little bumps on either side just below his pectoral fins that we noticed on the 21st, but then became much more pronounced mostly on one side by the 23rd. All around it and up into the fin is quite inflamed. The lump burst today, with white fuzz/puss? trailing out of a couple of spots all around the base of the dorsal fin.

(all of the other discus are fine so far, and the other fish too. One of the white butterfly tetra does have a small mass on its lower fin, but that does not look inflamed, nor has it grown)

Attachment 21160 Attachment 21161 Attachment 21162

What sort of pathogen usually causes this? I only have very generic medicines at the moment. I haven't put any rock salt into the tank because I don't know how much epsom salts are still in there and if this will be a problem..?

I have a smaller vacant quarantine tank set up and ready to go (mostly if he needs to be blasted with medication; since obviously whatever it is will be all through the main tank anyway). But just wanted to ask here what medicine would be best before putting him though a move or anything???