I have been there with this one. The leak is likely to be in the floor of the tank where a little water has lain.
It is not difficult to fix, just time-consuming.
*Pump the tank out until there is no diesel in it.
*Wash the tank again and again with washing-up detergent to get the diesel traces out of there, or at least minimised.
*You are going to need access to the floor of the tank.
*If you don't have already, you are going to have to cut an aperture in the top of the tank. In my case (a steel tank) I had to cut the top of the tank off. A grinder will do it.
*Be careful. You will have to flood the tank with water before you risk the sparks of a grinder.
*When you have access, scrub and scrub until the floor of the tank is clean.
*You should see where the leak is. If not, there is a trick.
*Now, flood the bilge with a hose until the level of water in the bilge climbs the wall of the tank. With a flashlight in there, you will see the leaks. Mark the leak(s).
*Pump bilge dry.
* Buy some two-part polysulphide adhesive... they normally are in tins. This adhesive is not cheap but it is a superb tank sealant.
*Clean the tank leak area.
*Apply adhesive with a wee roller.
*Let it cure... it takes a few days.
*Flood bilge again to test for leaks.
*If it is still leaking, place a coin to mark the leak(s).
*Drain bilge, let it dry, repeat application.
*Eventually, the tank will not leak whan the bilge is flooded.
*Pump bilge dry.
*leak test the tank by filling it.
*leave it for days to be sure.
*build an access hatch
into the roof of the tank and weld the top on again... the water must be in the tank or the weld spatter will burn the floor of the tank... watch your fire safety... beware radiated heat !!!!
*pump the water out.
*dry the tank internals.
If you can get the tank out, just make a new one, but if you cannot (I could not), then fix it in situ you must.