Black tank welding?

I would be concerned about the age of the tank. As they get old, plastic tanks of any material get brittle. I suspect the older they get, the less success you would have trying to weld them.

Seon wrote:
Years ago I bought an ABS welder from Harbor Freight that I never used.
Is this what you're looking for?

https://www.harborfreight.com/plastic-welding-kit-with-air-motor-and-temperature-adjustment-96712.html

I don't know, but looks like. How much you want for yours?
Tank is in very good condition. I tap the plastic and no signs of cracking - still very flexible even it is about 20 yo.
It is grayish plastic, no clue how to figure out material. Definitely no ABS I know from sewer pipes.
Since I'll be cutting some material off, can I make filling rod from the scraps?

I also have a harbor freight plastic welder and have done a fair amount of welding with it.
It takes a little practice, like regular welding, to get the heat and speed right.
I would certainly run a few test welds. You can use small strips of what you cut off to make welding "rods".
I found some good videos on u tube that helped quite a bit.

I order this WELDING KIT
It seems to be on small side, but I will have it tomorrow and no-hassle return. Than I operate in triple-digits temperatures all day, so have natural heat to help.
My tank only has a number written on it, what is not bringing any results, so still no clue what material is it.

Click For Full-Size Image.
Not bad for 1st such job in my life. Bear in mind I did work with torches before, so did not have to learn about keeping the torch away from the skin the hard way. ;)
The 700W welding station work slow in 90F weather, so for colder weather - more powerful one would be advised.
Main difference between metal and plastic welding is that you need to apply most of the heat to the rod and push melting rod into softened material.
Put the tank to 6" of water raise test and it holds without need for additional welding.