Help corals dying

[Heart is what matters]

Hello fellow reefers, im 5 months into the hobby I’ve been testing parameters regularly and corals were growing and thriving, just 2 days ago after a 20 gallon water change on my 75 gallon my alkalinity went from 7.5 to 9.8 used the same salt, water was at 78 degrees everything the same but shortly after the water change my hammer coral, goniopora, candy cane and some zoas that were all doing great fully closed. Just some advice as to what to do please. My sand starfish also died shortly after, Thank you
also feeding corals with reefroids and brightwell amino acids 2-3 times a week.


Parameters
salinity 34ppm
Alkalinity 9.8
calcium 473
Magnesium 1500
Ph 8.2
Ammonia 0
Nitrates 0 haven’t been able to raise nitrates even with more feedings
Phosphate at 15 “just added phosguard to reduce this.”

 

[Flippers4pups]

In cases like this, you need to test your newly made saltwater for dkh so you know what it mixes at.

How big of a tank?

 

[Heart is what matters]

In cases like this, you need to test your newly made saltwater for dkh so you know what it mixes at.

How big of a tank?

In cases like this, you need to test your newly made saltwater for dkh so you know what it mixes at.

How big of a tank?

Its a 75 gallon mixed, will corals survive to that big of a change or the shock will kill them? Some are doing ok like the gps, xenia,

 

[BrightReef]

If you've only had your tank set up for 5 months it could be the fact that it's not established enough to support the corals you have.