Since there is power already on the tank, why not use that same power to power up some small wattage spotlights mounted on the catwalk around the tank? Having the power up there also will mean there is conduit run from below, so one could pull another circuit up if needed. I am assumming that there is a catwalk, in order to service antennas and obstruction lights. We have several tanks lit this way.

Rick Miell