Slocke
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Thanks.Have you considered sending an ICP test? Just to make sure there aren’t any heavy metal build ups?
Activated carbon is my go-to when I get some coral deaths. It helps ensure toxins get removed and it can also remove organics and metals bound to organics.
Other than that, I would look out for predators or parasites. Besides for the magnesium issue (ICP is better at testing it), I don’t see anything else wrong parameter wise. The nutrients and all is OK. You can bring the alk around 7-11dKH, but 13 shouldn’t cause much death if it was raised gradually.
Oh, also test salinity and recalibrate the method of testing. Also watch for hidden temp swings. Too high can hurt corals IME.
I’m definitely having the new tank Alk bump I’ve always gotten. Might lower the AFR for a little while.
And I’ll definitely start running carbon for a while. Hopefully the new tank and the 90% new water has solved all the issues.