Question
hi i have a 2600 gl pond with 18 koi an gold fish i has a large swelling on right side the fish is 6 to 8 in an 3in around will this efect my koi an how to treat thanks for any ideas thanks harry

Answer
Goldfish often have problems with dropsy or kidney malfunction.  If the goldfish is grossly enlarged all around, that's what it is.  If it's a lump just on one side then that sounds like a tumor which is different problem.  Tumors can be made of fat, liquid, or rarely cancer.  They are not contagious.  Often, they go away on their own.  If not, they can be removed surgically by a vet.  If the fish does have dropsy or kidney malfuction, read the rest below.  If not, ignore it. I wrote it when I first read your question and then realized you said the swelling is only on one side.  If you want to send me a photo, then I can tell you more.

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Shubunkins are a variety of goldfish.  Koi can also get dropsy.  I have not seen a photo of one with kidney malfuntion.  With dropsy, the fish quickly becomes fat, and the scales stick out. With kidney malfuction, the fish slowly becomes fat so the scales grow with that and don't stick out; the fish eventually looks like it is going to explode and eventually it does (the cells all burst from excess fluid).  I have photos of fish with dropsy and kidney malfuction at
http://www.fishpondinfo.com/fishcare/dropphoto.htm and
http://www.fishpondinfo.com/fishcare/bigphoto.htm

Unfortunately, neither problem is really treatable.  You can try antibiotics or Epsom salts but I wouldn't use them in the entire pond but rather in a quarantine pond/tank.  The only thing that I've read can actually cure dropsy is an injection of antibiotics.  You'd have to see a vet to get that.

The good news is that these problems are normally not contagious in the normal sense.  The bacteria that cause dropsy are ubiquitous which means they're always there in the pond, and fish only get sick when they're weak for some other reason such as poor water quality.  You'll want to check your pH, hardness, ammonia, and nitrite levels to be sure they're ok.  Kidney malfunction is not contagious at all.  Good luck!