Consuming alkalinity but not calcium?

[gnikoli]

I've been experimenting with 2-part (Randy Recipe #1) as replacement for alk reactor and lime water. I've got a lightly stocked mostly SPS display with 240 gal total system volume so I don't expect to have a huge Ca demand. What do these numbers mean? Very little Ca consumption with disproportionately greater alkalinity consumption? Low consumption of both but Ca test kit (Salifert) not sensitive enough to pick up change in Ca? Something else like organic acids or CO2 using up alkalinity? My alk reactor was off so no CO2 from there, but house is closed up pretty tight although aerating water sample does not produce much of a pH change. Initial reading was taken several hours after adding supplements.

2/21 21:00 pH 8.16, Ca 375 ppm, alk 12.8 dKH
2/22 10:00 pH 8.00, Ca 385 ppm, alk 9.9 dKH
2/22 19:00 pH 8.14. Ca 385 ppm, alk 9.3 dKH

Thanks.

 

[bertoni]

A change of dKH from 12.8 to 9.3 is a total change of 3.5 dKH, which corresponds to maybe 25 ppm of Ca, which is probably close to the error tolerances of the Salifert kits, depending on how careful the testing is. I'm not all that precise, usually. CO2 will not change alkalinity, but there are some processes that will lower alkalinity but not calcium:

http://www.reefkeeping.com/issues/2004-12/rhf/index.php

Also, pH buffers add alkalinity, if one is in use.

 

[gnikoli]

Thanks for the comments and the link. In my case, I think that I just did not detect the relatively small change in Ca.