that is fairly new to have a fuel leak from any kind of corrosion. Did you check to see if there might be a screw in it? Does it leak at all fuel levels (full, 1/4 tank) Did you check all the fitting that come from the tank? Probably a valve that needs some adjustment. IF you can't see it maybe a little cutting may be in order
I have been part of the Mainship oewer's group in Yahoo since it's inception.
I don't recall ever reading a post about a leaky tank in one of those models.
As suggested I would check fittings first, then welds, then look for something that may have punctured it.
To find leaks I use blue work paper towels. Since its a new tank it might be a fitting or someing easy. Since the tank a so new it might be a poor weld.
Our middle tank slit so I cut an opening 18” X 18” to be able to craw in side. It was very easy in the dark tank to see the slit. I hired a welder who craw in side and weld up the split. Anyway the best fix is on the inside and use heat to burn off the diesel residue. However make sure you get ALL the diesel out before you put any heat to the tank and or try an out side fix.
My son works for a company that inspects large tanks inside petrochemical plants and tells me diesel can escape from a much smaller hole than water can.