Aquariums: Please help ASAP! Sick fish only getting worse. I don’t want to lose this fish.

I noticed one of my common goldfish had some kind of white growth on it's mouth, along with some fin rot. (The fish was born with a deformed mouth, it has a little harder time eating. It hasn't grown as fast as the other fish. Otherwise the fish was fine.) I had been having some anmonia spikes in the main tank, so I was doing more frequent water changes and looking for a different tank. So I put the sick fish in a quarantine tank, to prevent the other fish from getting sick.

I tried pristine water conditions for a couple days to see if there was any improvement in its condition. That didn't help. The white growth got a little bigger (kinda more downwards inside its mouth, you'll see in the pics how its lips go inwards) and the fish started shaking its head and holding its mouth open sometimes, like it was bothering it. It was trying to eat but was having more trouble than usual getting the food into its mouth. I figured it might be some kind of bacterial infection (cotton wool disease, mouth rot, ect.) so I started the fish on some kanaplex Thursday. Didn't see much difference in improvement, fish still was trying to eat if food was in front of it but it struggled. I gave it another dose of kanaplex today. The water parameters are all still good.

After about a day or so after I moved it into the quarantine tank, the fish started just kinda floating around in the tank, not a lot of swimming, just drifting around. A lot of drifting backwards slightly. Usually drifts in the middle/towards the top of the quarantine tank. If I turn a light on in the room or get close to the tank to look at it, it'll sink down towards the bottom of the tank. It still tries to go after food if offered but struggles and gives up when the food won't go in its mouth easily. It's not got its mouth up at the surface gasping for air, but it still looks like it's breathing heavier? It's gills are maybe too open/dark on the sides. I might be overthinking. It usually open/closed its mouth faster than the other fish, probably due to the deformity.

I've read the shaking of the head thing could be flukes, or some kind of parasite. I don't know how the fish would've gotten flukes, because I haven't introduced any other fish to them. Its just been them for over a year, unless they've been carrying them all this time. I found some seachem paragaurd today, and went ahead and bought it just in case. (There's no other parasite medication in stores around me, I'd have to wait days on shipping if I did actually need some). I don't know what to do. The other fish is doing fine, no white growths or fin rot.

Please help me. I don't want to lose this fish. What do I do?

More pics in the comments.