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Most angle iron is not um, ah, geezz I don't know what to call it. It tapers from the corner to the edges in thickness. This can make lay up more difficult. It would mean that without plywood the tank would actually be sitting on a very fine line all the way around unless you carefully angled it slightly.
A more major point is that angle iron is less strong than tubing for the same sizes. When you create a tube from a 'statics' point of view the tube acts somewhat like it is a solid bar the size of the tube. Angle iron works sort of the same but because it is only one sided it is nowhere near as stiff. U channel would be stiffer, then tubing is stiffer yet, for less material.
Steel ALL sells by weight. So for instance when you ask, "How much is this tubing a foot?" Someone looks up the price of the type steel the tubing is per pound. Then they check some table to see how much the tubing weighs per foot. Does some math and tells you X$/foot. You would need more steel per foot to do something with angle iron because of it's shape.
Another reason is that angle iron typically has a very rough finish you may need to deal with.
Also since it will be thicker it may mean you are welding a thinner tubing to a thicker angle iron which is a little harder to do, especially for a beginner.
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