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Carbon splitting plants
>When plants start using bicarbonate as their carbon source, I think the pH
>is usually higher than the 7.5 max in Alex's tank.
I agree that while plants are splitting carbon, the pH is likely to go a
whole lot higher than 7.5. BUT most low range pH test kits (used by most
non-rift lake aquarists) top out at 7.5.
>Also, as near as I can
>>tell from studies on the mechanism for bicarbonate use, plants shouldn't
>>decrease alkalinity when they use bicarbonate to supply carbon. The
>>bicarbonate is simply replaced with a lesser amount of carbonate that
>>creates the same alkalinity.
And what happens when the carbonates are exhausted? I _know_ that in
strongly lit tanks without supplemental CO2 (or even _with_ supplemental
CO2 but not enough) the KH decreases over time. In fact, Sera's whole
campaign is to let plants take advantage of this, and rather than add
supplemental CO2, just track KH and replace as necessary via their KH tablets.
If Alex's pH really _is_ going no higher than 7.5, then your explanation is
certainly a strong possibility, although my preference would always be to
lower fish and feeding loads and supplement CO2 rather than go to strong
aeration. Particularly if you are serious about plants. High fish loads
and feeding will inevitably lead to other problems down the line.