If you want to make your reef as eco-friendly as possible, then a few things to consider are using sunlight tubes to illuminate your corals, re-purposing containers to make your sump or refugium, reclaiming wood to make a stand, using solar panels to power your tank, and potentially even using rainwater in your system. The first several items from that list aren’t that out of the ordinary, but we know the suggestion of rainwater use will raise some eyebrows. While rainwater can contain many harmful chemicals (at least harmful to your aquarium), those can all be removed with a little filtration. Just set up a large barrel in your back yard that can collect the water, then run it through carbon media and some form of mechanical filtration. The water should be good, provided it is properly filtered. If rainwater is too much of a jump from the norm for you, then do what you can to collect the waste water from your reverse osmosis unit to water your lawn or to use it in some other meaningful way. The goal here is conservation and reducing costs.