You need to preheat the area 1 inch around the crack.  High temp heat gun.  closer to 1000 degrees F output the better. 

V the crack and scuff the area 1/2 inch around the crack.  Have some body hold heat gun in front of you and start putting wire in. Preheat the tank to around 450 to 475 degrees F right in front of the welder.   The wire should be 500 to 525 degree F  if you can get it that hot.  Not sure what the gun looks like but if there is no way to press the wire in then have somebody else use a utilty knife or handle of screwdriver to smooth out and press in the wire.  fill in the crack about 1/4 inch higher than the tank and taper it out so the whole "bead" is 1/2 inch either side of the crack. 

If you push threw the tank you are too hot!! lol

Course I have never messed with the "toys" for fixing plastic.  We have the "big boy toys" for that type of work.  Big over grown drills with wire grinders and heaters and a heat blow gun for preheating all in one.  Welds about 5 feet a minute with a big bead. 

 You need to scuff the area first or it will not stick.  NO MOISTURE what so ever and wear heat gloves cause things get HOT!

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