What you are generally encountering is the increased buffering effect of higher alkalinity within the tank.
As a general trend, the higher the alk, the less drift you get over a 24/7 period of the tanks pH
after increasing Alk...over several days you may notice the pH lowpoint (usually encounterd at the end of the dark period) will be slightly higher as the increased Alk limits the nightime pH suppression..it may well keep getting slightly higher over a few days as well, settling out to a smaller degree of drift overal. Equally though, you may find that your high point moves upwards slightly, becouse the tank is starting off each day at a slightly higher point than it had been previously.
it wont continue in this trend indeffinately though becouse the alk level (as long as it stays constant) sets the trend, and limits it (high Alk= small drift.....low Alk= large drift)
These are dirty numbers, but they illustrate the possible before and after effects.
Before..
Tank DkH 7.9
Ph High 8.3... PH low 7.95....Total drift 0.35
After
Tank DkH 9.0
pH high 8.4...pH low 8.15.....Total drift 0.25
The tanks total suppression factor is a combination of A: the bioload that gives off C02 at night and /or takes up 02 (fish, algae, corals, macrofauna, bacteria)..and B: the suppression affect from your Ca reactor. (the tank has to deal with the exess C02 released by the reactor via its effluant which is at a lower pH than the tank). there are other factors but these are by and large the main ones.
The trouble you will have with altering the pH of the reactor, is that you also directly affect the dKH output...If you increase the gas, you increase the dkH output. That affects the dKh of the tank, and subsiquently your tank pH again...If anything, lowering the reactors pH, will 'increase' your dKh, compounding the issue further..whilst raising the reactor pH will decrease Dkh output, and the level in your tank giving rise to a wider swing and possibly lower overal pH.
My advice would be to let the tank do what it wants to do...let it settle out at a pH its happy with...and use the reactor to control the tanks dKh, not the tanks pH...as long as your tanks dKh is within the right ranges, then all's well.
If anything, your in an evyable position...most people with Ca reactors actually have to fight a balancing act between adding enough C02 to get a good DkH out of the reactor, whilst not suppressing the tanks pH too much with large amounts of low pH effluant.
regards
Simon
