Assuming your Sailfish tanks are set up the same as on my Trophy Pro, the Aux is closest to the stern, the Main is closer to the helm.
(makes sense to have the larger, heavier tank amidships for stability and fuel burn efficiency)
If you have a Marina that sells VALVTECT gas, buy it. It already has all the additives you need to ensure against phase separation and other ethanol related issues.
Search here to find a marina near you:
http://www.valvtect.com/marina_search.asp