As steve and I have mentioned I can not see it being your aragocrete, if so you wouldn't be getting that low pH and he Alk would/should be higher. In a normal batch of IO the Ca++ is only around 380 and you are at 600. That is a rise of 220 ppm Ca++and I can not see that.
As far as CO2 and O2 the are not related. The suppressed pH can only come from 4 sources; overstocked tank, over feeding, dying life forms or high room air CO2.
Here is something on high room air CO2
http://www.advancedaquarist.com/issues/apr2002/short.htm
and here is something on Low pH causes and cures.
http://reefkeeping.com/issues/2004-09/rhf/index.htm
I'm not familiar with the Hagen kit but something is wrong with that reading or there is a piece of info we are not getting. The only thing would be if you were adding something like Kent Turbo Calcium but you say you have added nothing.
If I assume your readings are correct then one of the 4 must be happening
1. Test kit error or you are not using it right.
2. You have a bad batch of IO with high Ca++. I have never in 20 years seen this. 99.9% sure this is not it.
3. You are confused on the salt you are using, it is not IO but Oceanic, which has very high Ca++. I think you know what it says on the bag
4. The aragocrete did go into solution. It did raise your Ca++, pH and Alk but you never checked these for 2-3 weeks. During this time you had high room air CO2 and you or a child grossly over feed the tank or there was a large die off of life, like algae or combo of the two. I'm sue you will say no to all this.
Just so you know, this is not the first time you situation has come up. It has come up many times
.