Help me identify this disease please

[gasmaskdude]

Hi everyone,
I am new to this forum but not new to cory catfish.
Ive been raising and breeding catfish for over 3 years but some kind of new disease ive never come across has stumped me.
Tank details: 20 gallon with plants drift wood and decorations ; Substrate: white sand
Filtration: external canister filter and internal power head with air line
Tank has been cycled for a year, i do weekly 15% water changes and test parameters monthly.
Last test i did i got 0s for ammonia nitrate and nitrite, i use seachem purigen in my tank to keep it clean.
I use Seachem stability everytime i do a major water change and seachem prime .

So i recently got a new batch of corys for this tank, albinos ,emeralds and peppered cories. They came looking healthy but i did the usual 1 week quarantine before letting them into the tank. At the time the tank only had 1 betta in it.
Recently ive started noticing some white fungus/bacterial infection the dorsal fin on the emeralds and complete white coating on the peppered one. I did a quarantine of the infected fish. Use seachem paraguard for 1 week as per instruction with 1 hr salt dips on alternate days. The infections cleared up and fish were healthy although the peppered cats stopped eating.

Now they back in the tank again and have started showing same growth/infection but its more aggressive this time.

Only new thing ive used in the tank since starting it was seachem pristine.
Pics attached , any help would be appreciated.

Regards.

[gasmaskdude]

Update, the peppered ones died after not eating for a few days.
I have quarantined the rest again, they seem to be fine and the lumps are reducing.
I am considering doing a 80% water change, removing all gravel and boiling the ornaments and wood.
Any thoughts?