Get a turkey blaster or power head and clean the rocks.
If do you have a RR compartment, detach it so all the dirt goes to sump.
Detach your return pump and take the water out off the sump (with the return pump).
Check your fuge, shake your chaeto.
After all been clean, add a nitrate reducer from instant ocean ( you find at petco) to give a boost in your bacterias.
I was using the reef crystal salt, that it happened to me last month, I let the calcium been naturaly consumed, but when was time for water change, I got instant ocean (not reef version), and I am using until goes back to normal level.
I said for you move the sand bed, because it release natural and already balanced magnesium, strontium etc. At a momentarily small need I do this.
Anyway, I do agree that your magnesium may be low.
What salt do you use ? I would say try to steer your sand bed (that will release natural substances) that s gonna help the buffer work. _ I am not sure on that, but try.
I am in if do you get from reeftopia. They do have 100 blue legs crabs for $30, dozen scarlets for $36 and what I realy want is the peppermint at $2.
If someone is looking for fish, they have blue angel at $30 and other fishes at gret price.