When firing up the RO/DI unit after it has been sitting for a while, be sure to toss out the first couple of gallons, as they may be nothing more than water that has stagnated within the filter. Even though the water coming through the filter is supposed to be clean, it can still sit in dirty filters long enough to become a real harm to your reef aquarium. So, unless you have water flowing through the RO unit constantly, we highly recommend throwing the first little bit of water into the drain so that your tank will get only the best. To prevent bad water in general, be sure to also change out the filters and backflush the membrane regularly. This will ensure that you always get the best water in your tank.