Black Goldfish Lump

Question
Hi. I have a black goldfish (with the bulgy eyes), that has developed a large
lump on its side. The lump is making my fish float at the top of the water on
that particular side. Its making it hard for it to eat, as it can only float and has
a hard time getting moving from the top of the water. It remains hungry
though.
It recently had tail rot, and I was treating it with Maracyn II. The tail seems to
be doing better as it looks as if it is growing back to an extent.

I change 15% of the water about every week and a half, and change the filter
and carbon as well. So I don't think this is related to poor water quality. I have
another goldfish in the 10 gallon tank, and it is doing fine.

Any suggestions on how what the lump is or how to get rid of it would be
greatly appreciated. I think I have to do something or it may starve.

Answer
Hello Chad,
Firstly, yes this particular issue may or may not be due to poor water quality, but two fish that are both supposed to grow over 6 inches long and live for twenty years should not be put into such a small tank. You should have at least a thirty gallon tank if you want to keep two goldfish together. They grow much too large, and produce way too much ammonia to live for any length of time in a small tank. Goldfish are very long living - the oldest recorded at 80 years of age - and are not even adults until reaching 3 or 4 years of age. So please work on getting a larger tank as soon as possible.
Secondly, doing only 15 percent water changes in a small tank with two very messy fish (eat a lot and process their food much quicker than most fish, meaning they make a lot of ammonia very quickly) is not acceptable. You need to be changing out at least 30 percent once or twice a week. Maybe more - depending on what your readings are for ammonia and nitrites/nitrates. Have you been testing your water weekly? Chances are you have high levels of ammonia. This is toxic and dangerous to fish - and ALL fish produce it. Some more then others - goldfish included.
Lastly, it sounds as though your fish may have SBD. Stop feeding them for 48 hours. Then, at the end of those 48 hours, give your fish a small amount of deshelled, crushed pea. This helps aid in digestion. From now on, cut back on how much you are feeding. A fish's stomach is not much larger than it's eye!
If you are feeding flakes, it's a good idea to crush them into tiny pieces, soak them, and then feed no more than a pinch or two a day. This will greatly reduce the chances of your fish getting SBD, which goldfish are very prone to.
Fish are much more complicated to keep than most people think! That being said, I wish you the best of luck and keep me updated if your fish does not recover within two or three days. First things first - change out a good portion of that water - pronto!
p.s. Your goldfish is called a Black Moor, for future reference.