Actually you could just put both electrodes at the bottom of the tank and place an open tube above the hydrogen electrode to keep the hydrogen from mixing with the tank water. The bubbles would all rise through the same small column of water and exit at the surface.

I just thought it was relevant to the original post, I have no intention of buying it. I'm actually building a PSA oxygen concentrator as we speak that should produce (up to) 120 L/min of 95% pure oxygen.