Drilled out the rivets holding the tank drain in place. The drain had sealant around it but for some reason the sealant had not adhered to the tank skin resulting in the leak.
Removed the access plate. Cleaned the access plate and drain. Riveted the tank drain into place with new sealant. Replaced the access plate.
Puzzled by the fact that looking through the drain hole there was a lot of light in the tank. Inside the sealed tank. FUDGE. Sealed a flashlight inside it.
Removed, cleaned, and resealed the access plate. No light inside the tank this time.
