Assuming for a moment the tanks are say 2' tall and you want the bottom one a foot off the ground and a foot above the bottom tank, that puts the top of the top tank at roughly 6'....I would spend the extra money on steel, especially since the water alone will total 4,000 lbs. 2,000lbs of water plus tank and decor at eye level will require considerable strength and wood is going to need to be carefully considered. Standard 2x4 construction should be upgraded to 2x6 and back bracing designed in. 500 gallons on display will be awesome.