What is going on with this angel?

[Hazard5]

Hello,
I need some help please. One of my angels started breathing heavy about a week ago. I moved it to a quarantine tank. I noticed a couple days ago red marks that developed near the mouth. I treated with API general cure. Next dose is due tomorrow. No improvement yet. The fish is swimming and eating fine. QT set at 80 degrees. I removed the carbon filter in QT.

My main tank is 150 gallon with a fluval FX canister filtration. It has been running about a year now. Water parameters all normal. I check with API water kit. PH a bit high at 8 or at least approaching 8. My local water source is always high. Temp at 80. There are 6 angels, 8 rainbows, 2 roseline sharks, a couple algae eaters, and a parrot fish.

I've had fish for a long time and I have never seen this before. The rest of my fish appear fine. I'm worried this is something that may pass from angel to angel. Has anybody seen this before? Thank you for your help?

 

[Aiken Drum]

Please take a read through the "unhealthy fish" sticky and give as much detail as you are able.

https://www.aquariumadvice.com/forums/f17/before-posting-about-unhealthy-fish-read-this-32451.html

How old is the fish? Are you seeing any aggression between you angels? If they are about a year old same as your tank they will just be starting to pair up and establish territories which could cause some friction.

 

[Hazard5]

For clarification the filtration is a Fluval FX4. I do weekly water changes 20-25% with gravel cleaning. Last changed this past Saturday. I've had the angel 8-9 months. They are not aggressive or started pairing yet that I can see. I feed them New life spectrum optimum flakes and pellets, with Fluval bug bites and frozen brine shrimp. Occasionally some freshwater mysis. Water parameter checked yesterday. Ammonia 0, nitrites 0, nitrates 20, temp 80 and PH around 8. Nothing added too recent, one fish added about a month ago. The angel is breathing rapid for last week. Almost like there is not enough oxygen in the tank, but I know that is not the case. No injuries that I can see. No spots, fungus, bloating, red marks on rest of body. Fins look fine. I noticed the red areas around the mouth a couple days ago. It's possible it's been there longer and I just noticed it in the smaller QT. It looks internal. The rest of the fish are unaffected. Thank you!