Young tanks / cyling / establishing biological beds eats alkalinity, and can eat it really fast. They need to get carbon to grow from somewhere. Also, live sand and crushed coral contains some organics, and as these decompose (they go from a sealed bag to lots of fresh 02 in salt water) they use up alk.
Short form is alk is used up by all kinds of biological processes, especially when tanks are cycling. I woulnd't even bother test calcium and magnesium. It's not going to go anywhere and you are just wasting test kits. I would also target a higher alk value like 8.5 or 9 if you are going to balance calcium at 450.
Use a capfull of ammonia vs that dead shrimp. Saves you a lot of time.
I'm ussing dead shrimp now. I can't find any ammonia solution here in the Netherlands that I trust.
Do you know a good solution I can get here