[Aquattro]

Just in case this helps anyone, here is an excerpt from Sanjay Joshi's paper.

"There are basically two adjustments that can be made to an operational reactor — the effluent flow rate and the amount of CO2 added. In the steady state, the trick is to balance the output of the reactor with the daily consumption of alkalinity in the tank.

Increasing the CO2, while maintaining the effluent flow rate, results in a decrease in the pH of the reactor and hence an increase in the solubility of the CaCO3 media, and hence a higher alkalinity in the effluent. This increase in alkalinity of the effluent will be seen up to a pH range of 6.3 to 6.5 and any further decrease of the pH in the reactor will start resulting in decreasing alkalinity. Increasing the effluent flow rate, while maintaining a fixed amount of CO2 will result in an increase in pH in the reactor and hence a reduction in the alkalinity of the effluent. By adjusting both the effluent flow rate and the CO2 injection rate, a fixed pH can also be maintained in the effluent. Some advocate maintaining a fixed pH of around 6.5 in the reactor.

I find it much easier to adjust the reactor based on the alkalinity output rather than the pH measurement. I would recommend first setting up an effluent flow rate so it flows in a continuous steady drip, and then making adjustments to the CO2 flow rate to increase or decrease the alkalinity of the output. For most tanks this approach will work fine, but if you have a heavily loaded small-polyped scleractinian (SPS) coral tank, then it may require you to increase both the effluent flow rate and the CO2 injection rate. A certain amount of fine-tuning is required to adjust the reactor for your particular system.

Having a good estimate of the daily consumption of the alkalinity in the tank and understanding some of the “reactor math” can help in eliminating some of the trial and error in fine-tuning the reactor. Let us assume that the reef system contains T liters of water, and the effluent flow rate is L liters per hour and the estimated daily alkalinity consumption is c milliequivalents per liter (mEq/L) per day. Now, measure the alkalinity in the tank and the alkalinity of the effluent. The difference between the two values will give you the increase in alkalinity due to the reactor — call this d (mEq/L) — as follows:

alk/day added due to the reactor = (d x L x 24)/T Equation 1

So, now we need to adjust the reactor so that the daily increase due to the reactor is approximately c mEq/L. This will give us the setting at which the reactor will replenish the alkalinity that is consumed daily.

Looking at the Equation 1, we can see that there are three ways this can be achieved:
Only adjusting d — the increase in effluent alkalinity
Only adjusting L — effluent flow rate
Adjusting both d and L.

The effluent alkalinity can be increased (or decreased) by correspondingly increasing (or decreasing) the amount of CO2 and keeping the effluent flow rate constant. This provides one convenient way of tuning the reactor output to the aquarium needs. When increasing the amount of CO2 added care must be taken to keep the pH level above approximately 6.3. I personally use this approach to adjust my reactor. If I find that I have to injected too much CO2 so as to cause the pH in the reactor to drop below 6.3, I am better off also increasing the effluent flow rate through the reactor.

Increasing the flow rate will result in a decrease in effluent alkalinity if the CO2 flow rate is not simultaneously increased. Several manufacturers recommend adjusting both the flow rate and the amount of CO2 simultaneously to maintain a constant pH (about 6.5) in the reactor and hence a constant alkalinity output in the effluent. I prefer having to just adjust one parameter — the CO2 flow rate. Both approaches will satisfy the needs of the user, but the key is to balance the daily consumption to the daily addition of alkalinity."