Tank is crashing :(

[iceman86]

Hey guys, maybe someone here can help my figure out this crash. My tank is going through a crash right now. All the fish are fine but I'm losing all my corals (sps and lps). I've a ready lost some large sps colonies and at least a dozen smaller ones and a bunch of frags. My sps started bleaching some from the top, others from the bottom and others from the middle. My lps aren't many (mostly acans) half have died and the others look fine. My Monti caps are even getting random bleaching spots.

I have always used t5 since the beginning of this tank but in January I switched to led to get away from the hassle of changing bulbs. Everything looked great or about a month and then the sps started receding where it was being shadowed. And getting random burn spots. I figured it was the fixture not being full spectrum and not enough spread so 2 weeks and I got rid of that fixture and replaced it new ones that are full spectrum and will cover the tank more so I don't get any shadowing and also added some t5. Things kept going downhill and dying.

300 gallon tank actual water volume
Alk 8
Cal 470
Mag 1320
Po4 0.04
Nitrates undetectable
Potassium 390
Sg 1.025

I've also replaced the cal, mag, and alk kits to confirm my readings.

I was using a doser for years and worked well but I switched to a calcium reactor in January to make things a little easier. At that time I had the parameters dialed in 8 alk 420 cal and 1350 mag. Tested for a couple weeks and held stable so I only tested alk from that point. I tested last week and alk was 8.7 but cal was 490 and mag was 1320. I'm back to dosing to bring the calcium back down before I turn the reactor back on. I just tested at 470 right now.

I've replaced carbon and gfo often, keep filter socks clean and do all the maintenance to keep it clean and nothing changes.

All I can think of is that in January I bought some nori for my tangs from Walmart. I used it for about a week but I didn't like it because it would almost fall apart in the water. I then looked up the ingredients online and there was some oils that they added to it so I went back to feeding my old nori. It almost seems like my skimmer hasn't worked right since then.

Also around January my rodi filters got neglected and the d/I was exhausted. I'm not sure for how long but I'm sure some stuff got through the filters. I've replaced everything since then.

I've also checked for stray voltage and didn't get any.

Only thing I haven't done is a water change due to the large volume of water needed to make a difference.

Any ideas guy?