Sick goldfish-turning black and upside down

[meli]

Hello,
We have had a goldfish for a year now and everything was going fine until 1 month ago. Now he is covered in black and he is pretty much upside down, he can move his fin but he is not able to swim properly (he is basically carried by the current). We do water changes regularly and we get our water from a fish store. I don't know what to do anymore, I think he is in a lot of pain, but I cannot bring myself to kill him. Is there anyway to save him? He has been sick for so long, I am beginning to lose hope. We do not have a water quit, so I am unsure of the readings.

 

[Delapool]

Well I typed a bit and lost it so this will be shorter.

Hi,

Why is there change water being used from the store?

Where did the black start on the fish. Tank details and stocking? The black could be chemical burns - pick up the API freshwater master test kit to check ammonia and ph. Are you using a water conditioner?

If you have methylene blue, do a single half-dose into tank. This will help with low-level infection cleanup + breathing.

Methylene Blue | Kordon!

Did you introduce anything to the tank a month ago?

Hope your fish pulls through.