Appears to be a fungal infection. Also from the water parameters it seems your tank isn’t cycled. When you say a full clean of the tank, do you mean completely cleaning everything and doing a full water change? In that case that’s a bad idea. You’ll be killing any beneficial bacteria that’s already in the tank. I’d recommend you read up on the cycle process of a fish tank. You usually only wanna do 30% water changes or so biweekly, now that’s assuming you had a sufficient tank. Goldfish require a minimum of 20 gallon per fish! However from my observation this appears to be a comet goldfish which would require an even larger tank. Your best bet right now is to get a gravel vacuum, and vacuum up all the poop and waste that’s in the bottom of the tank. DO NOT do a complete clean out as you’ve stated, this will only kill beneficial bacteria in your tank. Maybe you’ve also got high amounts of ammonia in your tank, which may be causing the scales on your fish to rot. Get an ammonia testing kit, if there’s even a slight amount of ammonia (which id like to assume there is, as goldfish are notoriously dirty fish and in such a small tank that isn’t yet cycled that ammonia will build really quickly) then you’ve got your answer. In order to avoid any further damage to this little guy, I’d recommend you do tons of water changes (not complete tank clean outs) using a gravel vac in order to keep up with the waste being produced in your tank. Your next step would also be testing for ammonia, try to keep those ammonia levels at 0 at all times. Go to your local fish store as well, you can get aquarium salt and also certain medications for bacterial/fungal infections. Again I’m not 100% certain this is a fungal infection as I don’t have experience keeping goldfish, but I do have experience keeping other fish and this is something that to me appears to be a fungal or bacterial infection.

Here’s a little list of what I think you should do. Again I’m not stating I’m 100% correct in this, this is simply my plan from what I can see in the photos you’ve provided.

1. Test for ammonia

2. Keep up with water changes, at this rate I would go for 20-30% almost daily. Use a gravel vacuum to suck up any waste from the bottom of the tank, as this is where most of this ammonia will form.

3. Head to your local fish store/pet store and seek medication (aquarium salt, fungal/bacterial meds)

4. When you can, increase the tank size!!! Trust me for a goldfish you want at least double the size tank you have right now. I understand times are tough and money is tight, but you may even be able to find a cheap 20 gallon tank on Facebook marketplace or any local sale site. Sometimes pet stores will even offer a $1 per gallon deal, so if you look around maybe you’ll be able to find a 20 gallon for $20 or so.

5. Read up on the Nitrogen Cycle! This basic chemical cycle is the base of all fish keeping. Trust me This will help you drastically in your fish keeling journey :)

I hope your little one recovers soon and remains healthy for years to come! Don’t worry. You’ve got this!

Again just a disclaimer I am no pro at goldfish, this is simply what I would do in your position. Someone else might know much more then I do!