Help with sick emperor angel

[kbarr31]

I suspect my emperor angel might have velvet but am not that experienced and wanted to ask for opinions . He’s been in my tank for about 4 months now with no issue and I’ve had the rest of my fish in my tank for about 9 months with no issues as well. I recently added a fish without even thinking and without QT him and 2 days later he died and now I think my emperor has velvet. His one fin is pretty small looks almost like it was eaten but I know it wasn’t he’s got these weird patches on his tail and fins and on this one area of his skin around his gills he’s still eating and relatively swimming well besides an occasional laying between the rocks for a rest which he doesn’t usually do. I haven’t had any issues with the rest of my fish. Is this velvet ? Or ich or something else? Any opinions also best treatment he is in a reef tank with mostly LPS and a few mushrooms. My tank parameters are

Calcium- 565 (which I know is a bit high)

Magnesium- 1,480

Nitrate- 3ppm

Alkalinity- 8.7

PH- 7.8

phosphate- .4

Temp- 78

 

[Uncle99]

Body shot looks velvet-ish, but pics can be deceiving. Super fast killer if true.
What other symptoms or changes in behaviour?

 

[kbarr31]

Body shot looks velvet-ish, but pics can be deceiving. Super fast killer if true.
What other symptoms or changes in behaviour?

He’s still eating but can tell he’s a bit more lethargic sometimes lays down for a few seconds other than that I don’t see any other symptoms besides his fin on the left side basically being gone and the weird patches on his body and fins, any suggestions for treating velvet in the fastest and best way ?