In an ideal world we would have a probe that measures Calcium, carbonate alkalinity, pCO2 and pH feeding to a microcomputer that forecasts trends of where the ca and alk were going, and how the pH of the system would affect these two variable parameters depending on CO2 consumption/liberation by photosynthesizers, but I doubt that we as hobbyists will see such a device in our lifetimes at a price that the average hobbyist could afford (except possibly, Jerel's Dendrochronologist...
).
If you use the tank pH to control the CO2 solenoid, it will shut off the supply of CO2 each time the pH of the system drops below whatever bottom level pH setpoint or rises above the top level setpoint you use
regardless of what the cause is for those shifts in pH. Normally pH in an aquaria would be driven by the concentration of CO2 in the aquarium (pCO2
tankwater) And controlled to some extent by the amount of bicarbonate/carbonate buffering available to the water column. While the CO2 is running and the effluent is at an appropriate flow rate, then alkalinity will continue to be added to the system and pH will not shift too low for the system
so long as the CO2 bubble rate is correctly set overall (not too excessive) or the effluent pH is not too low. The idea behind using the aquarium pH level for the CO2 controller is that MOST situations in stable systems where the pH drops below 8.0 or so are caused by using too much CO2 to dissolve the reactor media inside the reactor. This results in the excess CO2 ending up in the aquarium, dropping pH to unsuitably low levels. If the controller set point is set for a low of 8.0, then the controller shuts off the solenoid when the water column pH falls to 8.0. The reactor will stop releasing any more CO2 for the water column
but this also means it will stop the manufacture of Calcium and alkalinity buffer.
This is fine so long as the excess CO2 was the cause of the pH drop, but for the sake of argument, what if the pH is dropping due to the presence of excess uneaten food, or an undiscovered dead fish...
...behind the rock...
...that died the day you left for a long weekend with your spouse/boyfriend/girlfriend/nextdoor neighbor/dog/cat/dendrochronologist (you get the picture
)
...and you're gone for 3 days
As these substances decompose, they generate both free protons and organic acids that in-and-of themselves will both deplete your alkalinity and drop your tank pH as the buffer is consumed. In this scenario, the probe for the controller is set for 8.0 as the cut off point, and if the event occurs at night, you pH is already at the 8.0 level in the morning. A viscious circle could ensue that ends up killing other specimens, which provide more fuel to the organic acid fire, which drops more pH units, which kills more specimens... ...but the reactor stays off with the pH of the tank dropping further and further below 8.0, so none of the alkalinity is replaced as it is consumed by the organic acid protons.
Tank crash ensues and pH can end up around 4 or so...
An extreme case and unlikely, but possible when the effluent is not used to control the rate of CO2 delivery to the reactor. Setup in this manner is a lazy means of avoiding titration of the reactor (dialing it in) to produce the amount of Ca and alk necessary to replace and maintain these parameters in the water column. With the current level of reasonable available technology, you would ideally have the effluent set up to control the level of the reactor effluent pH (via CO2 solenoid control), and a plain pH monitor to read the pH of the water column with the probe placed just upstream of the effluent for the CO2-based Ca reactor (and the Kalkreactor if your system is so equipped). Placement of the tank reading probe should be such that it is not in the tank, but at the point where the water column is returning to the sump, but far enough away from the reactors that there is little chance of feedback in the sump circulation (preferably on the other side of a divider where the introduced sump water must go over a wall to reach the main sump area). In this manner, you will still have a visual means of detecting your water column pH while assuring adequate pH of the effluent. This will provide you with the max amount of alk/Ca while using the min amount of CO2. Some systems may use a second column of reactor media to increase the pH via neutralization/reaction of the excess CO2 (especially in systems with high rates of effluent flow and heavy Ca/alk demands). Expect effluent pH to be closer to the 6.9 to 7.0 range in these systems after the second column. If you continue to have pH issues after cleaning and recalibration of the probes and consistently low pH, Ca, and alk readings, then a second column may be necessary for your reactor. The use of limewater may become necessary in such systems if demand still exceeds capacity for delivery after adding the second column, but look for improper adjustments if your system does not have large populations of Calcium consumers.
Different reactor media will require different effluent pH readings, as will different equipment configurations. ARM should be at 6.7 to 6.8. Calcite (much more dense) may require pH as low as 6.5. Some soft oolitic limestones may only require 6.8 to 6.9, I do not have numbers for high magnesium dolomitic limestone, but personally do not recommend it anyway as it skews your conservative element's proportionality. Shells in the media will release silicates and phosphorus, and are to be avoided. Too fine a media and the reactor will not flow well, too coarse and it will not have good cotact time for dissolution. Some systems may benefit from a recirculation collector for excess CO2 in bottom up configurations, or reversal to top down if excess CO2 gas becomes a problem. Any lower than recommended pH for each media and you start burning up the media, often resulting in the entire mass turning to a slushy muddy mess. Much higher, and you aren't getting enough Ca or alk from the water going through the reactor. One method of removing excessive CO2 (albeit wasteful) is to have the effluent return into a cylinder set in the sump, requiring that the effluent collect in this cylinder and overflow into the sump for the final return to the water column. An air stone may be added at this point to aid in the degassing of the effluent, but this may raise the pH enough to begin precipitation of the effluent when concentrated in the cylinder area. Best to allow for the immediate dilution of the effluent by having the flow rate set to deliver directly into the sump water in a
high flow area. Use the
needle valve to control the rate of CO2 delivery rather than degassing the effluent, as not only does it prevent system pH drops, but it conserves your CO2 as well.
If you need a means of placing the probe in the effluent prior to dilution in the sump, build a cylinder of 2" diameter PVC pipe with a cap on the bottom end and a "X" fitting at the top. Run your effluent into one arm of the horizontal "x", place the probe in the top opening, and make the other horizontal arm the exit for the effluent going to the sump. Extend the incoming pipe to the bottom of the container to assure thorough displacement of the contents as new effluent enters the test chamber.
I am sure that I've left something out here, I've been making sure that everyone gets their homework done and that the misc critters are taken care of today. Post any concerns you may have and I'll try to get to them tonight.
sorry for the long winded response
HTH
