Since when do we store gasoline. I thought we burned it up pretty quickly.
If you are worried about water in your gas for say your lawnmower, get yourself a Mr. Funnel. It has a teflon membrane inside the funnel that seperates the water from the fuel as well as seperates any rust or crud. All that goes out the bottom of the funnel is pure gasoline or diesel if you pour in diesel. I have one and it works as stated.
Being a farmer, I have my own storage problems but it's with diesel. I have a 1000 gallon bulk tank that I fill yearly. I have to use a biocide in the diesel to prevent algae from growing in the water that mixes in the diesel from tank condensation. Injection pumps are way more expensive to rebuild or replace than the fuel delivery system in a gasoline engine.