Update:
New sump plan was going well, until the problem with the Alk Doser hit.
I was wondering why I had some Cyano, and corals looked a bit annoyed.
My Alk dosing pump had jammed and I did not notice. I need to test water more often,
but when things are stable for a long time, you get lazy.
Alk dropped to about 5.5 dkh.
And interestingly, Calcium went way up, to about 600.
I have been reluctant to add Alk back very fast, with that high of a Calcium level.
Good way to get a snow storm of precipitation.
Plus I want to avoid PH swings.
So lots of water changes and extra Alk added every day. Getting better.
Using Instant Ocean is a benefit : A bit low Calcium, high Alk.
But I definitely lost a new-ish small SPS and a torch coral. Some others are not happy.
Time will tell what else happens.
Fortunately fish do not care.
Doser is temporarily fixed, but planning on replacing that.
(I created a different thread on which one to buy)