bodeba, I've been using three products to maintain my water chemistry and I'm very happy with them. I use Seachem Reef Buffer (to get the water to 8.3 on a one occasion basis according to instructions.) So, that's what you're looking for, I'd think. Once the water's at the 8.3 that you want, I find subsequent addition of that not to be necessary.
It may raise your carbonate hardness a little so you have to make sure you test using a reliable test kit. I understand that you don't want to raise alkalinity but this won't be a problem for you.
Once the pH was at 8.3, I used and continue to regularly use Seachem Reef Builder in conjunction with Kent Turbo Calcium to maintain the balance between calcium and alkalinity. I refer to these "gadgets" regularly for the dosage necessary to maintain the chemistry where I want it to be:
http://www.reefnerds.com/ChemPlot/ChemPlot.html
http://home.comcast.net/~jdieck1/chemcalc.html