With all my respects , reading Tooch81 answers my feeling is that is a little confused with the marine system.
As been written.....step by step.
First step: have the correct water quality. And I am not thinking in Water to keep acroporas,
Second step: take out the suspected bad crab, but there is no need to take all the crabs.
As it was written :
Alk : 7-9 dkh (achieve 1 value and get it stable). To make it easy : "with a low number of corals and being most of them soft and LPS a 10% weekly water change will keep your Alk in line with the salt you are using. So NO NEED of thinking in dosing Alk, Ca or Mg".
Ca: ~420 ppm and Mg will be replenish with the Water Changes, like the Alk and Ca.
Phosphates and Nitrates : keep them low (Phos below 0.05 ppm and nitrates below 5 ppm) those numbers are general indicators of good quality water
The marine tank is not rocket science. Is very easy if you follow some husbandry and rules.
Snails die from time to time, also in the most stable tanks. Zoas can be closed for weeks and they are alive, just not happy, but alive.
You will see that after you stop doing changes and just keep the basics the system will start to work. Example, DO NOT CHASE PH. John K wrote it very clear. Moreover, when you measure something try to do it at the same time. Example: ph in the morning is at its lowest level and increase during the day. In the late afternoon its at its highest and start to decrease after light are off. The ph level, as John wrote very well, depends on many factors. The most important is bicarbonate (Alk) and photsynthesis (CO2 levels). Do not chase it, as John K wrote.
Start to buy good test kits. Salifert are cheap and excellent. Measure your Alk, Ca and Mg levels in you tank and in you new salt water. Compare, they have to be very similar with such low bioload you have. Post those values and we all can continue discussing.
Cheers
Daniel