Well I have decided to take advantage of the added lumens in my grow tent. I have 2 six hundreds in a DRII 120 and began looking into CO2 generators. I have a tank system I scrounged up, just waiting on the monitor/controller to show up. But I began thinking of an easier ways and more cost effective ways instead of exchanging/refilling my CO2 tank. While doing research it did not take long before I started thinking. What is the difference between a water-cooled CO2 generator such as a HydroGen MiniGen Water Cooled CO2 Generator and an On Demand Hot Water Heater for example a Marley Tankless Heater? Links to both: HydroGen: hXXp://www.hydroinnovations.com/product-details.php?pro=3 Marley: hXXp://marey.com/productDetailsi.asp?productID=9 They both seem the same to me:confused2: Except for the price and the Marley has a higher btu/hr burner. Do not know if the heat exchangers are similar or not. Does anyone know or have experience in this?