As I'm sure you've already gathered from reading previous posts in here and doing outside research you already know that 2watts/gal is the minimum for growing plants. For a deep tank like yours, a good deal more than that would be helpful to make sure enough light penetrates to the bottom. The best way to get that much light in the space you have available on the top of a hex tank is with metal halide lighting. I don't know of any ready built MH systems that will fit on a hex tank, but if there are any, I'm sure there is another member in here who will offer a link to where you can find it. If you find yourself having to make one on your own, a number of sites offer the component parts: ahsupply.com and hellolights.com are just a couple. Do a lot of reading before you buy. Lighting is very important and can be very expensive. When you get it right I think you'll be very happy. Big hex tanks like yours are beautiful things to behold when done right. Good luck.