I had fungus develop on my juvenile motoro ray's tail previously. The fungus started right at the sting, and first spread downwards to the tip of the tail before spreading upwards towards the disc.
I treated first with Salt (didn't work) then Melafix (didn't work) then with Furan-2, which partially worked. What happened was the fungus on the disc and upper 1/2 of the tail subsided...but then the rest of the tail from the sting (where the fungus first originated) fell off!
So now my poor stingray has 1/2 a tail with no sting, and showing no signs of growing it back!