Discus tail ballast leak

A friend of mine has a Discus cx with a leaking fin tank that has been difficult to remedy. When viewed from the fin bottom, the water does not appear to leak around the dump valve but from somewhere else. The maintenance manual has a schematic of this area. Does anyone have a suggestion as to how to gain access to the hardware just beneath the tank or perhaps there is a Discus owner group that could assist? Most discussion of tank issues relate to its wing tanks.
Thanks,
Dee