Low pH & KH on my 70gal reef tank

[FozzyBear]

Hi guys,

I've had my 70gal. tank running for 18 months now, slowly adding corals and fish in time when I felt the opportunity occurred. The water parameters have been steady, ever since I decided putting the first corals in (4 months after initial startup, initial start 22 months back.)
Ca constant between 420 & 450ppm.
Mg > 1320ppm.
Density: 1.024-1.025.
Temperature 25-27°C (yeah, its degrees over here, not farenheit).

My question is this: I can't get my KH steady above 7.0. It seems to linger about 5.4-6.2.
pH is also steady on a rather low 7.8.

My gut feeling tells me pH should be around 8.0 - 8.2 and KH around 7.0 - 9.0.

Is this a problem? I've been offered dozens of KH enhancing products and they all raise it for day, maybe 2 but then it falls back. Its frustrating, expensive and giving me the idea these stores don't have a clue.

I've got a lot of stony corals which are fine. Does anyone think a Ca reactor is overkill on a 70gal. tank? Some suggest it's too much CO2 in the water but I DON'T have a Ca reactor!!!

Any chemists around?

[Reefing Madness]

Calcium, KH, GH, pH, Electrolytes & Magnesium in Aquariums; Mineral Ions, Cations
Only thing I have for you. It has some great info there for you.