Before I answer, Are you running CO2 injection ? Try taking a sample of water, leaving it in a cup overnight and measuring the pH again. Does it change ????
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I wouldn't be worried about it to much, in fact 6.8 is the Ideal pH for most aquatic plants as it offers the most balanced pH for nutrient uptake....
pH decreasing is a fairly common phenomenon in heavily planted tanks, there are a few causes, but without going into too much detail:
- Acidic substrates such as aquasoils will reduce pH
- So will, Fish/Bacteria (Most living things) as they produce acidic wastes...
- Plants will consume KH as a source of carbon (In the absents of adequate CO2) and by effect, decrease you pH
The solution is easy, add more KH.... Increase your KH to 5/6'dKH ( >100 ppm) and this should make a difference.
Raising you KH will generally bring your pH up and stabilize it there.
See below for more details about this relationship:
[CLICK HERE]
For planted aquariums, I suggest using a Potassium based KH buffer as opposed to the traditional sodium based bicarbs.
The potassium based buffers increase you KH and Potassium (Beneficial for the plants) simultaneously.
In rare cases where raising the KH does little to the pH, especially in older established aquariums with pH controlled, CO2 injection. Addition of a strong base, usually KOH (Potassium Hydroxide - Strong, instant pH up) is used to neutralise built up organic acids in the tank and restore pH without adding excessive amounts of KH. This is more of a specialist case and would required a few pages worth for a full explanation. Note that KOH in "shelf" concentrations must be handled with care and dosed carefully as it can induce potentially fatal jumps in pH if used incorrectly.
In conclusion, add a couple teaspoons of KH generator and that will bring you pH up.
For most planted tanks, aim for 5 to 6 degrees KH = 107ppm
Hope this Helps,
Rovik.
