URGENT!!! Goldfish unknown disease, PLS HELP!!

Hi all, this is my first time posting to a forum so please be patient w me
Over this last weekend one of my three fish, a large-ish lionhead goldfish, deteriorated rapidly over the course of about a day and had to be euthanized by about 6pm. It started in the morning when I noticed that pieces of his tail were falling off and he was a bit lethargic, but other than that nothing different. I thought it could be fin rot and went out to buy some aquarium salt. By the time I got back he had deteriorated much further; he could barely swim and his scales were flaking off. His fins were also bloody. He was so weak that he got caught in the filter (this was a first since the filter is relatively soft and he's big). I tested the water and the pH was high but other than that, nothing was off. I dosed the aquarium with salt, turned off the filter, tried to get him to eat. Nothing was helping and he looked like he was in so much pain. I put him down with clove oil because there was just no way he was going to recover. Poor little guy, I miss him. The decline happened so fast.
I currently have a 29 gallon tank with one bristle nose pleco and one goldfish. I know this pair isn't optimal but they haven't had any problems since I got them, about 3 years ago. The pleco has not been aggressive at all.
Yesterday I noticed my little guy's scales looking a bit worn, and his fins looking bad with some white on the ends. It looks like the beginning of whatever my late goldfish could have had.
I've scoured the internet for what it could be but I'm coming up blank. Parasites maybe?? He's looking significantly worse than he did yesterday, with scales flaking off and his fins looking more worn. I know I don't have a lot of time to find a solution. Luckily he doesn't look weak yet, but he's been staying my the heater so I turned the heat a little bit up for the entire tank. The pleco looks fine.
Parameters: ammonia - 0ppt
nitrate - 0 ppt
nitrite - 0 ppt
pH - around 6.5 (this is low for the tank but it's because I dosed the tank with pH lower due to its pH being too high)
29 gals, air stone, plants, snails, 25% water change every 3 weeks
I've never seen anything like this and can't find anything online. I love this little dude a lot. Please help!!! Idk what to do!!!
(btw: all the white spots on his body are missing scales)

it sounds to me like a blood infection, septicemia, which is generally the late stage of earlier internal infections. Even with the best of care, it seems to be what happens with growing goldies in a tiny (to them) tank. Eventually, with the amount of waste produced by goldfish and plecos in a tank not ideal for even one of them, some sort of infection takes hold.
Goldfish and common plecos start out easy to keep, but they're actually very difficult fish because of their space needs. 3 years means you have done a good job with them.
I took care of a group of goldfish overwintering in a Canadian school with a too shallow pond, and I put their 75 gallon beside a sink, because I had to do so many water changes every week. They produce a lot of waste.

GaryE said:

it sounds to me like a blood infection, septicemia, which is generally the late stage of earlier internal infections. Even with the best of care, it seems to be what happens with growing goldies in a tiny (to them) tank. Eventually, with the amount of waste produced by goldfish and plecos in a tank not ideal for even one of them, some sort of infection takes hold.
Goldfish and common plecos start out easy to keep, but they're actually very difficult fish because of their space needs. 3 years means you have done a good job with them.
I took care of a group of goldfish overwintering in a Canadian school with a too shallow pond, and I put their 75 gallon beside a sink, because I had to do so many water changes every week. They produce a lot of waste.

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Ok, thank you so much. I’m picking up some broad spectrum antibiotics ASAP. Hoping they help. Should I add aquarium salt as well or hold off? Also, should I isolate the goldfish so that the pleco doesn’t catch it? Is the tank’s water now infected?

If there is blood on the fins of any of the fish, or streaks on the body, then the infection is probably too advanced. I would do 50% water changes every day for a week, then if the fish survive, two or three times a week forever.
Salt probably won't do much if it's an internal infection.

If you don't have a second larger tank to isolate the goldie in, it's better where it is.

Dolphianity said:

pH - around 6.5 (this is low for the tank but it's because I dosed the tank with pH lower due to its pH being too high)

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What is the pH of your source water?

Fluctuating pH (caused by pH adjusters, KH buffering and source water differential) is very stressful on fish. It is preferable to leave the pH alone. Goldfish/ black moors don’t need acidic water, neutral is fine. Bristlenose plecos are ok with acidic or neutral water (5.5-7.5 for common ancistrus sp.).

The pH adjusters and resulting fluctuations can cause stress resulting in disease, often even more so than living in water with the ‘wrong’ pH. If the pH is genuinely not suitable, RO water would be a more stable way to rectify it.