As said there is pressure testing that needs to be done properly. There is also some design considerations for structural integrity with regards to baffle placing and such. And using only aluminum fittings, no brass like some manufacturers cheap out with.
I found it to not be worth it to make tanks and I make a living bending and welding aluminum on boats. I really can't beat the price of coastal in Bellingham when you add in the cost of the aluminum, fittings, sending units, bending and welding, rate of decay pressure testing, etc. I can buy their tanks built to whatever dimensions I need and have them delivered for cheaper and not have to worry about the certification stuff since they all are certified. They build for many boat manufacturers around here.
That said I've built them for my own boats
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