I have that exact one. The gauge doesn't work, but I am getting bubbles from my 3 in 1 diffuser. I use that and a hagen nutrifin in a 65 gallon tank. It has been 2 weeks, and I see growth. My angel/baby tears are dying, but my dwarve angel tears are definately growing. My Co2 long term tester is greenish blue, but not the green that it recommends. A side note is that I use a large air pump for my sandfall, and a HOB filter.
The ebay seller has been reluctant on sending me a replacement gauge. It looks like everything else works fine though.