Leaky Alu MIG welds

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I recently made a 120 litre rectangle shaped fuel tank for a friend using 3mm aluminium sheet.  Normally I would use TIG to weld it up, but seeing as I have spent my money on a MIG with twin pulse, I thought I'd give it a bash.  Welds looked great, but there were 23 pin hole leaks when I pressure tested it. In the end I made another one using TIG. There were 3 pinhole leaks which were easily fixed.  When I tried to fix the MIG welded leaks by melting it with the TIG, it seemed that there was some sort of inclusion, and I was unable to stop the leaks.  What do you guys think might be the cause?
Reply:Where on the welds were the leaks?   Starts/stops?  Joint intersections?  At the toes or in the middle?     I welded watertight aluminum frames with pulse mig in my old production job and the times a pinhole would happen were most likely at a start/stop, or jucture of 2 joints (corner).   Other times were if the weld got off to one side of the joint a bit and had a lack of fusion or lack of fill on one side, or from moving the torch too quickly and not getting good preheat, fusion and fill as the weld was progressing, or from running too hot and popping through.  Sometimes there would still be a pinhole after repairing the blown-through area.An inclusion or porosity would indicate dirty metal or lack of shielding.MM350P/Python/Q300MM175/Q300DialarcHFHTP MIG200PowCon300SMHypertherm380ThermalArc185Purox oaF350CrewCab4x4LoadNGo utilitybedBobcat250XMT304/Optima/SpoolmaticSuitcase12RC/Q300Suitcase8RC/Q400Passport/Q300Smith op