Well, these lumps on your goldfish aren’t actually “warts,” but it’s the best descriptive term we have. Most of these growths are benign cutaneous tumors within the layers of the fish’s skin. (Technically nerve sheath tumors aka neurofibromas or shwannomas.) It is very common to have goldfish with a few lumps, bumps, nodules or growths; whatever you want to call it.
These growths are not of concern to the overall health of the fish. They do not cause any quality of life issues other than aesthetic ones. Sometimes, these growths can get very large, and due to resistance from a swimming fish, fall off and grow back. They tend to be highly infiltrative into the skin, so surgical excision is only a temporary fix. We have attempted to apply cryotherapy to several cases, with good temporary, but not long-term, resolution.
Mario, the goldfish pictured above, had multiple growths on his body and fins. This image was taken a few weeks after cryotherapy. The attachment of the growth caused the top portion of his tail to require regrowth, but you can see the lumps are not truly gone.
So, these lumps on your goldfish are really nothing to worry about. There is the potential for them to become abraded and require topical care, and they may fall off and grow back, but your fish’s quality of life will not be affected.