Very sick Angek

Sick angelfish https://imgur.com/gallery/fcAnkCn

Ive posted on the cichlid sub, but figured that here would be appropriate as well. So here's the backstory.

I had an angel pair. The male died from jumping out of the tank a few weeks ago. Earlier this week I introduced a new male angelfish into my already established tank. He was fine for about 3 days then start exhibiting weird behaviors. Gulping air at the surface, being lethargic, rolling over. Then withing 24 hours he passed away.

I thought maybe i just had bad luck as all of my parameters are good and what they've always been.

Fast forward to today my female is now acting weird. Lethargic. Spending most of her time at the surface. I noticed that she has a raw-looking spot on top of her head. Im not sure what this is. Ive come to the conclusion that the angel that was introduced earlier in the week must've brought something with him.

55 gallon densely planted tank. With co2
pH - 6.8-7.0
NH3 And nitrites are 0
Nitrates fluctuate between 0-40. But have been 0 over the past few weeks.

I ruled out co2 gassing has my indicator showed that i was within the non threatening range of dissolved co2. Plus none of the other fish are experiencing any issues.
Those being about 10 cardinals, 5 cory cats, and a few nerites.

All in all Im looking for possible treatment options. Ive been doing frequent 50% water changes/cleanings.