My father-in-law recently gave my wife and I a 20 gallon aquarium that came with a koi (it is a koi, right?) that is about 6 inches. While the aquarium may be able to support the fish (using a general surface area rule), I intuit that the fish really should be in a larger aquarium or pond.
I've had aquariums in the past, but this fish seems to swim quite fast for the limited area he has to swim. Is this an indicator of stress? To make matters worse, it appears that the fish has several diseases. The most apparent disease (or maybe it is a result from mistreatment) is that there are many scales on the fish that appear to be missing, but when the light hits them just right, it looks like they are all there. It's almost as if the scales are transparent have not retained their shininess. Also, on its right fin where it meets the body, there is a bumpy sore. Furthermore, on a few of the transparent scales and looking closely at the fins you can see small capillaries that didn't seem to be there when we first got the fish.
I've checked the ph of the tank, and it seems to be on the very low end of the scale (acidic). I've changed half of the water once (the ph of our tap is neutral) but seems to have had no effect on the aquarium ph level. Could the low ph be causing these diseases? Can ph be corrected? What can I do to address the individual diseases?
I would like to be able to sell or give the koi away so that I can put more appropriately sized fish in the aquarium, but I figure I will have a hard time getting rid of a fish with diseases. Moreover, I feel kind of bad not being able to help heal the fish. If the fish does regain it's health, how do you recommend I get rid of it?
Here's a picture of the fish:

Here's a picture of the fish in the 20 gallon aquarium:

I apologize for the quality of the pictures... my wife's camera is not that great. Anyway, thanks for your time and I hope you can help!
I've had aquariums in the past, but this fish seems to swim quite fast for the limited area he has to swim. Is this an indicator of stress? To make matters worse, it appears that the fish has several diseases. The most apparent disease (or maybe it is a result from mistreatment) is that there are many scales on the fish that appear to be missing, but when the light hits them just right, it looks like they are all there. It's almost as if the scales are transparent have not retained their shininess. Also, on its right fin where it meets the body, there is a bumpy sore. Furthermore, on a few of the transparent scales and looking closely at the fins you can see small capillaries that didn't seem to be there when we first got the fish.
I've checked the ph of the tank, and it seems to be on the very low end of the scale (acidic). I've changed half of the water once (the ph of our tap is neutral) but seems to have had no effect on the aquarium ph level. Could the low ph be causing these diseases? Can ph be corrected? What can I do to address the individual diseases?
I would like to be able to sell or give the koi away so that I can put more appropriately sized fish in the aquarium, but I figure I will have a hard time getting rid of a fish with diseases. Moreover, I feel kind of bad not being able to help heal the fish. If the fish does regain it's health, how do you recommend I get rid of it?
Here's a picture of the fish:
Here's a picture of the fish in the 20 gallon aquarium:
I apologize for the quality of the pictures... my wife's camera is not that great. Anyway, thanks for your time and I hope you can help!