Stable means having the same ppm of CO2 in the water during the lights-on period every day, day after day. You can't use a drop checker to be sure of having that. First, it reacts so slowly to changes in CO2 concentration that it makes trying to adjust the CO2 bubble rate by watching it is futile. Second, it is very hard to judge the color of the drop checker, so what looks like the same color could be a 2-1 difference in ppm of CO2 in the water. The way to achieve stabile CO2 is to adjust it once, then leave the adjustment the alone until something in the tank changes enough to justify a change in bubble rate. Shut off the CO2 when the lights are out, using the solenoid valve, and turn it back on a half hour or so before the lights come back on, again using the solenoid valve. But, don't touch the regulator or needle valve adjustment.