welding plastic resh water tank

Repairing polyethylene or polypropylene can be problematic. The age is definitely a factor as the plasticizers tend to disappear either due to chemical bonding or evaporation leaving a brittle material. It can be welded with a heat gun but you have to be very careful about temperature as it will go from tacky to very soft quickly. A backing material to support the soft plastic would assist the procedure. If you need patch material to build up the repair site getting a compatible melt index is close to impossible, unless your able to cut off a piece of excess material elsewhere on the tank. I do not know of a satisfactory glue for use on poly. You might be able to fix using cover plates with 5200 and riveted together.
I have repaired holes and cracks in plastic kayaks but its ugly work. And additional cracks eventually recur.

pops