Unexpectedly high alkalinity

[GMay]

I have an unusual situation with my alkalinity. My newly mixed water tests at 7.3 dKH and is as expected fro the brand of salt I use. When I test the water in my tank it tests at 9.3 dKH. I am not dosing alkalinity or anything else into this tank because I do two 10% water changes a week and it have vary light bioload (3 small fish and two small zoa colonies) in a system volume of 18 gallons. I am seeking ideas about where the increased alkalinity may be coming from. I started the tank just short of a year ago and used Reef Saver dry rock.

 

[W1ngz]

What are your nitrates at? Alkalinity can also increase from the de-nitrification that happens in bacteria and photosythesis.
I am assuming you are mixing the water yourself, using RODI that tests 0 TDS?
Please confirm that the only thing you add is food. Nothing else, whether you do or don't consider it dosing (some people have their own idea what dosing means).

Here's what I would do to check a little further.
Test your alkalinity right after a water change.
Skip the mid-week water changes to get a wider test range.
Test your alkalinity right before the next water change in a week.

 

[nereefpat]

Adding any "buffer?"

 

[GMay]

Adding nothing but food for the fish.
NO3 and PO4 are both below the below limits of detectable for the test kit. I have used Red Sea and Salifert kits.