Well, I've been running my Midwest Aquatics XL Denitrator for a couple of months now. Great piece of equipment IMO. But keeping that drip rate has proved a bit of a problem. I even bought a pinch valve from Marine Depot that's "Not supposed to clog", but it hasn't been perfect for sure. Sometimes my nose will lead me to check an odor of sulfur and sure enough, drip rate dropped. But, I've yet to see any "black".