Aluminum melts at a lower temp if that helps any, it will also scratch or score because of its softness where SS under normal conditions should not. Darn I haven't messed with stuff like this in 30 years, Hard to remember. Don't you guys in Canadia have a Non ferrous metal magnet???
Sure looks like aluminum to me. Check the welds, SS has a small bead pattern where aluminum is almost like holding a seagull and squeezing it out its rear thick and heavy. But a good welder can make al welds small also.
Dont tell the guy to place a magnet on it. He is from Kanada and only has a picture of this tank on his computer. He will try to place the magnet on his screen and it will ruin his computer screen.