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

More than likely it is ABS. That means you can solvent bond it. If you can find some ABS sheeting, you can use ABS pipe solvent and glue the patch over it. That will be much stronger than welding, and you are much less likely to get it too hot and damage the molecular structure.

I have welded an ABS tank, but I was using an ultrasonic welder, which allowed me to press the rod into the crack. That worked very well, but I had a lot of control of what I was doing, much more accurate than a hot air welder.

Harbor freight tools had a plastic welding kit I have always wanted to get one, have not had the need yet I hear they work well I have only seen pvc welded with it though. good luck

I have never done business in the area you are in, but you want to get on Thomas net. The companies you want do not generally do business with the general public. So, they won't have websites or yellow pages ads.

The tank is probably polyethylene and it can be welded if it has not been in the sun.

Matt