Black Moor Goldfish has large lump/growth on back near top fin, what could it be

I have had a Black Moor goldfish for 2 years now in an Eclipse six aquirium. I change the water regularly (every 3-4 weeks). The fish acts very healthy: eats fine, swims fine, very active. However, about five months ago a lump/growth started to grow on his back near his top fin. I didn't think much of it at first, but it has been doubling in size over the couple of months. The lump is now the size of a grape and my fish is a little bigger than my thumb, so of course the lump looks very bad.

Lump details: it is the color of his scales (black), when it grows in size I notice whiteish openings then they turn black after a few days. I also notice the lump seems to grow at night when the tank light is shut off... very weird. It is has a lumpy texture.

Can someone please help?!? I have tried using Ick Clear, E.M. Erythromycin, and Maroxy, but none of these products have cleared up the growth. Even when the tank has been cleaned the lump continues to grow.

It could be a tumour and the cause could be environmental ( eg. chemical leaching into the water ) or viral infection, it could be inherited through it genes but I doubt so in your case.

It could be Lymphocystis see this site http://www.tropicalfish4u.co.uk/acatalog...

http://www.aquarticles.com/articles/mana...

or lastly it could be Nodular disease casued by parasites. http://www.fish-disease.net/diseases/nod...

http://www.thatfishshop.com/health/nodul...