white lump on caudal fin w/black streak??

[ceruleanblue]

Edit - we were able to put him in a jar to look at it closer - it is attached between the 2 caudal fins where they meet, it is 1/4 inch or so big, quite clear in color, the black I am seeing is the tips of his fins that this is attached to. Looks a lot like a blister?


Hi,
I have 3 fantails in a 20 gallon cycled tank that have been in there for at least 6 months, parameters taken right now are (sorry I don't have any more test kits):
ammonia 0 ppm
nitrate 20
nitrite 0
The goldfish all have been healthy and eat well. Today, tho, I just noticed Willie has a white lump or patch right inbetween where his/her upper caudal fins come together/meet next to his/her body. There seems to be a small black streak in this lump or patch. I'm not sure what this is. Can anyone help or have any ideas? We are in the middle of a blizzard so the only medications I have for at least the next day or two is:
Melafix
Quick Cure for Ick and Parasites
Maracyn_Two for Dropsy, Septicemia, Popeye and Fin and Tail Rot
T.C. Capsules for Gill Disease, Cotton Mouth, Bacterial Tail Rot
MarOxy for true fungal infections and bacterial diseases
Parasite Guard for external parasites

I can quarentine him but is that necessary?
We're very rural and can't get to a pet shop easily. I was told these are "general" medicines I should have around just in case but I've never had to use any yet.