Originally Posted by Whitebeard
The area aroung the drain was damaged, probably from a road impact. It looks like someone tried to plastic weld it but did a rough, sloppy job and there is a small leak in the repair area. The tank is dark gray.
If the leak is small, and the weld is holding OK but sloppy (very common), it likely there is some extras material around the small leak or nearby. The tank is non pressurized so only sees the water head pressure, so super strong not critical. I have made similar, ugly, plastic welds and I also would get small leaks so I think your issue is similar.
What I usually do is clean it all up well, and maybe rough up the leak area bit and pick out any dirt that got in. Tank has to be empty and totally dry in the that area. Make sure that no added sealing attempts have been made as any silicone or other stuff will make it nearly unweldable without grinding it all out.
To fix the hole, use a soldering iron to melt and push some surrounding material into the leak area and fuse it in. You need to get hot enough to melt and fuse, but not overheat the plastic. I have found this works nearly all the time if no other products have been used to try to seal the area. Even if it doesn't work, you have little time in it and haven't wrecked anything.
Others may know for sure, but the tank is likely polyethylene and almost nothing sticks to it well.