IO has been known for a low Ca
reading, Oceanic for a high Ca
but low pH. Kent has been all over the place but my last two buckets were very low in Alk
/pH and Kent found the same in the samples I sent them.
Have you checked your magnesium level yet.
You may need to supplement with something like Kent's superbuffer AND turbo-calc (calcium chloride) till you get a balanced/stable reading.
I have now switched to Tropic Marin which give me better readings on my new mix. I'm still supplementing with Turbo Ca
for my corals.