I've had this goldfish for around 2 years now, and have no problems with it at all. I changed it over to a bigger tank (22 litre) around 3 to 4 months ago, and until now have had no issues. The tank has NO other fish within it, and just some standard plastic trees and gravel.
Every 2 weeks I change the water (around 25%) and also use "fish safe" water conditioner to try and remove some of the impurities within the water. I also use a manual vacuum to clean the fish gravel.
I'm unsure whether I've just not noticed, or whether this has happened in a small time frame, but the back fin on my fish is starting to look a bit ragged.
The other fins appear to be fine; I thought the black marking may have been a sign of poor water quality, but they seem to have been like that for over a year now.
Additionally, it seems to be swimming in one corner of the fish tank, which is very unusual for this fish as he/she is usually happily swimming around the whole tank.
I have given it some "disease safe" product, but as I'm unsure whether this is an actual issue, I don't want to overload the tank with lots of chemicals it doesn't need.
I've attached some images below to help clarify what I'm referring to.
Would any fish/pet experts be willing to offer some advice?