Magnesium= 900

my conversion of meq/l to dkh is a little poor, but that means the alk is around 14 dkh? That is a little high (i like to keep mine around 10-13) but I wouldnt be too alarmed by that level. This might be resulting in you having a harder time keep mag up. Are you dosing any type of buffer? I would hold off on that for a while and monitor your alk until it gets down to 11 or so. But start dosing mag while this level is going down. There is many types you can use, seachem mag (powder form), kent mag (liquid form), brightwell (powder) and others. You can get it bulk at bulk reef supply.

Also what is your SG at?

 

I don't dose anything. Everything in my tank comes from the salt. My Alk is currently between 6-6.5 meq/l (16.8-18.2 dkh). It was holding steady around 4.5 (12.6 dkh) for a few weeks (down from 21 dkh).

Salinity is at 1.027. pH is now about 8.1.

 

You should drop your SG down to 1.026 at the most.

 

It typically is below that. I'll top off with unsalted water tomorrow to bring it back down.

 

Do you top off with saltwater at times? You don't want to do that since salt doesn't evaporate.

 

Nope. Never.

 

to get coraline algea, get some scrapings from anothers tank and dump it in front of the powerhead.

 

I've done that twice. The one rock with coraline is not as dark a color purple and the scrap marks from 10 days ago are still white (hasn't been grown over yet).