Lump on Fantail

Question

Fantail   We have a generic fantail goldfish that is over seven years old. (she is the youngest in our tank.) On the attached photo you can see a large black growth; it is continually getting bigger. I think it appeared about 6-7 months ago. Do you have any idea what it might be and how to treat it? Because of where it is located it affects the way it swims. Sometime the fish seem fine, other times - like today - it appears to be limited in motion.
Thank you for your response.

Answer
I am afraid that is a Goldfish tumor or cyst. Cyst are rather harmless but I am leaning more toward the accusation of a tumor as they seem to immobilize the fish.

Unfortunately there's not a big cure for a tumor/cyst unless you want to risk removal often times this is caused by genetic defect, exposure to high levels of toxic water parameters for long periods of time.

Even if you risked surgery, by putting the Goldfish under a small amount of clove oil, and cutting it off with a razor or surgical scalpel it could simply come back in month's time or you risk a bacterial infection with an open wound.

If you see anymore symptoms caused by the tumor/cyst I recommend humanely putting it down.