The fact that this is such a new tank, and you're adding both calcium and buffer, leads me to think that you've got your parameters out of whack and your calcium is not staying in solution. At this stage in your tank's life, there's probably not enough coralline growth to really require ca
supplementation other than normal weekly water changes.
Based on that thinking, I'd lay off the buffer/ca
, wait a couple weeks doing your normal water changes, and then retest things at the end of a couple weeks. Then you'll probably have to follow the advice given in this article...
Chemistry and the Aquarium
... to get your alk
back in line.
Not familiar with the tetra kit, but have used the API calcium kit. While I've found it's not the most accurate in the world, it DOES seem to be pretty consistent and get you close enough for what we need it for. So I'm thinking your test kit is OK.