Goldfish has a swollen base?

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Please help!
A 4 year old goldfish of mine suddenly has his tail base all swollen! The scales are raised, but only there and nowhere else on his body. This happened over night. He still has appetite but he swims a bit "drowsy" and I can clearly see that he is not feeling alright
A month before he also had a swelling/pineconing around his whole body, I treated him with antibiotic food and then, he quickly became well again. Now, to my dismay, I woke up to see this!
My tank is a 200 L aquarium with 3 goldfish currently inside. The water parameters are:

Nitrite: 0 mg/L
Nitrate: 25 mg/L
GH: 10 dh°
KH: 10dh°
pH: 7,6

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Today is the day where I would normally do a water change, so the parameters will become better.

Could anyone help me out here? This fish is really precious to me, since I bred him by myself

Thank you in advance

 

Bump. I hope someone else has some answers for you. He's a beautiful fish.

 

Do you have an ammonia reading as well? Anything besides the goldfish in the tank? Any decorations he might have injured himself on?

 

Do you have an ammonia reading as well? Anything besides the goldfish in the tank? Any decorations he might have injured himself on?

Ammonia is 0, I just tested it.
Besides goldfish, I have some snails in the tank (mainly Brotia herculea)
I do have a piece of wood in my tank and I saw that he has a cut. He probably cut himself and thus, injured himself

 

Oh no Well maybe it's a localized infection and is treatable! Do you have another container you can quarantine him in for easier treatment?

 

Oh no Well maybe it's a localized infection and is treatable! Do you have another container you can quarantine him in for easier treatment?

Yes, I have a 50 L quarantine tank in case I need it.

 

I am very sorry everyone. The fish in question sadly passed away...
I am super sad right now...

 

I'm sorry for your loss x