Okay I got results for you!
RODI Water : 0 CA, 0 ALK, as expected
RODI doing its job.
My Tap : 90 CA, 9.0 dKH ALK
Seems like your water is harder then mine. Also Seachem Prime, is only to neutralize chlorine from the water. Its not meant to lower alk or ca levels.
If your in this hobby long term, I highly recommend getting RODI unit and start making your own water. I used to buy water from my local LFS and my tank never did well. Random fishes died, I could never keep corals for very long. So I invested in some test kits and found the RODI water I been buying was pretty much tap water. Their salt water had some crazy nitrates and phosphates. Salinity levels were always different! So I had to mix more salt or add more water. After my RODI unit, I know exactly whats going into my tank and has helped me keep things alive.