@detc I have heard that algae cal has strontium which can give false results on the dexa. I am not an expert at all - but please look into this before investing in these.
Separately, I have a problem with taking calcium and my dr advised that I only get Calcium from food sources. There is a great food based Calcium called 'Seen' which has 500mgs of Calcium per 'cookie' (made of milk and dates). I highly recommend this vs calcium supplements in any case as the body will absorb food sources better.