I use different test kits which read in degrees for kh and gh but from what I understand 120ppm of kh is 6.7 degrees of kh.
With that kh and ph, according to the kh/ph/co2 correlation charts you only have about 7.1 mg/l of co2 in your tank.
The goal is around 30 mg/l. So you want to get your ph down to around 6.8.
You may not have enough bps, or you may have too much surface agitation.
In my opinion as long as you aren't running into algae issues, your fish are breathing and acting normal and your plants are growing quickly, hitting exact levels of co2 is not completely 100% necessary.
Ph controller is not as necessary as it is a safety net that will keep your co2 from suffocating your fish. It also is only efficient if the way you are diffusing the co2 into the tank is efficient. For example if the bubbles are just floating to the surface and escaping your co2 will never turn off, and you will never reach optimum levels of co2.
Surface agitation should be kept to a minimum if not yet done. It should be a slight ripple, but not splashy of any kind. Once that is accomplished if co2 levels do not rise, up the bubbles per second. Make sure you do not leave the house after messing with co2, observe the tank and livestock.
Additionally the flow is very important, you should be attempting to keep the co2 bubbles in the water as long as possible to maximize the amount of co2 being diffused into the water. The only difference I see between regular diffusers(glass/plastic/chopstick/airstone/etc.) and in line types is simply that you have an easier time aiming the flow of where the co2 bubbles actually go. On the other hand a reactor type diffuser actually increases the efficiency of co2 being dissolved into the water.
It seems there is some debate as to which type of diffusion is more efficient for plant growth. Co2 bubbles sticking to plants or no bubbles and adequate co2 levels in the water. Both seem to work anyway, so it comes down to if you want to see bubbles or not. Reactor types seem easily more efficient but with the right flow and an in line reactor I'm sure you can have the same results.
Last edited by theericafish; 06-10-2013 at 07:34 AM.