Help to diagnose coral issues :)

[SamBackspins]

I have a 40 breeder reef tank with a 20 long sump. I added corals after 2ish months and have slowly been stocking up to now, with it being around 6-7 months old. Until now, all my corals have been doing fantastic, I've seen great growth and color. Approximately 2ish weeks ago, all my corals closed up and started dying. A decent size colt coral I had was the first I noticed, and it died. All the Zoa's, my Goni, just everything. The trachyphilia is holding up alright but still looks bad regularly, even the mushrooms.

As far as I can tell, my parameters are all doing pretty good, I have a good flow which varies, coralline algae has EXPLODED in the tank, and theres so much of it growing on glass and pumps, the coral is placed in the places the flow is good for and the light as well, I just can't figure out why on earth they wouldn't enjoy their time, especially soft corals. Nothing really changed either?

Tank and Stock Specs
2 Nicrew Hyper Reef 50W LEDs
1 Mars Aqua 165W LED
20 Long sump, with live gulf rock as the primary media
UV Sterilizer (AA)
Multiple heaters keeping it at 81 consistently.

Livestock
Wheelers Goby w/ Tiger Pistol Shrimp
Royal Gramma
2 Ocellaris Clownfish
Yellow Banded Possum Wrasse
Red Stop Light Cardinalfish
~20ish hermits
~15ish snails
~2 emerald crabs
Gorilla crab (in the sump, I think he came on some live rock, but he's cute in the sump so)
Porcelain crab (also in the sump, only have seen it once or twice and he hides too much to ID him, probably a porcelain crab but I dont know)

I pretty much consider myself fully stocked and don't want any more fish, besides maybe another cardinal.

Tank Parameters
Alkalinity: 8 almost exactly
pH : 8.1
Phosphate : 0.1
Calcium : 420 ppm
Magnesium : 1280 ppm
Ammonia : 0 ppm
Nitrite : 0 ppm
Nitrate : 1-5 ppm (hovers in this range pretty much constantly)

I'm just very lost, I've dealt with dinoflagellates so far, they're gone, I noticed hydroids a while back and they're gone now, all the fish are incredibly happy, they're never really shy, I EXCLUSIVELY feed mixes of frozen food, Mysis, Brine, LRS, Reef Frenzy, and some other stuff. I just don't actually know where to start.

Heres some pictures too. Thanks in advance for any help.

 

[danieyella]

Parameters stayed consistent? No big drop or increase that then stabilized out? Any temperature swings you know of? Did you do anything close to when you lost the first one? A water change, a new addition, move a powerhead, change a dosing brand, etc?

 

[SamBackspins]

Parameters stayed consistent? No big drop or increase that then stabilized out? Any temperature swings you know of? Did you do anything close to when you lost the first one? A water change, a new addition, move a powerhead, change a dosing brand, etc?

I tried dosing alkalinity and calcium together for the first time somewhere around when the first one was lost, but my parameters stayed pretty similar honestly. In the past two weeks, I've been trying to adjust various things like powerheads and everything to see if it fixes it. I'm also very slowly switching from Red Sea normal salt (not black bucket) to Tropic Marin, I replaced my GFO, I added Cuprisorb to combat heavy metals because I found a rusted clip in the sump. That's about it, the salt change I'm doing fairly slow as well.

The parameters I listed have been pretty representative of the parameters for the past 4 months.