I drilled holes in my water tank by accident and successfully repaired them so I did all the research and know exactly what you should do. You are going to weld the holes shut using material from the same tank. Nothing will stick to PE plastic except melted PE so using some shavings from the same tank is preferred. There are different grades and blends of PE so don't take a chance with the bond strength and use plastic from the same tank.
If the tank is covered with insulation then chip away what you need to to gain access to the leak location.
Remove thin shavings of plastic from the tank corner(s) or edges with a sharp wood plane. The plastic in these areas is very thick and taking off a little bit won't compromise the tank at all. I got this idea from the tank manufacturer.
With a plastic welder (hand-held soldering iron with a flat tip-available at some hardware stores or Harbor Freight) melt and work the melted material down into the hole and then flatten the hole area with the flat tip.
I removed my tank to fix the holes because they were on top. It was really easy to do. This repair is very easy to do and requires very little skill...I did it! Good luck!