Tank installation advice
Another recommendation for Luthers Welding in Bristol, RI. I had twin tanks made by them a few years back. As far as installation is concerned, it isn't the tank getting splashed or occasionally wet that you are worried about. What you does aluminum tanks in is anywhere that moisture is trapped against the tank with no way for air to dry it out. Anyone that has removed an aluminum tank can attest to this. The corroded spots are always where it sat on the stringers, or was against a bulkhead, etc... This is why you don't want the tank sitting directly on anything that can trap moisture. The recommendation is to take plastic strips and glue them to the tank using an adhesive like 5200. Be generous with the adhesive. Now the tank will sit on the strips, with no way for water to be trapped against the tank. Any water that gets on the tank will simply dry off. When you attach the strips, be sure keep in mind how the tank will sit in the boat - for example, if you were attaching to the vertical side of the tank, putting the strips vertically would make more sense than running them horizontally- less chance of water puddling.
An aluminum tank with airflow around it will not have corrosion problems.
Just don't use rubber against the tank- allegedly it is cathodic to aluminum.