Clam sudden death, zoas are not opening. What's going on?

[evolutionZ]

Hey fellas, so here's my problem.. Everything was fine till about one week ago, when I noticed some of my higher end zoas starting to close up. I usually get that once in a while so I didn't give it much thought. I was busy with my exams too. So yesterday, my blue crocea which I kept for 4 months suddenly closed up and within 6 hours, it died. It was totally fine the day before. I noticed more zoas closing up and some had a little algae on them. They also looked like they are melting.. My green brain didn't swell up yesterday too. The weird thing is that all my sps (over 20 frags and colonies) were totally fine.

No major changes were made. I started using Randy's 2 part but ca and kh was stable and good, my mg was 1200 2 weeks ago and over the course of 2 weeks I raised it slowly back to 1320. I started using zeovit coral vitalizers about 2 weeks ago at 2 drops a day for my 65g.other than that no other changes.

My water parameters as of last night :
Ph : 8.4
Sg : 1.025
No3 : 2ppm
Po4 : 0ppm
Ca : 450ppm
Kh : 8
Mg : 1320

All were measured using salifert test kits.

Anyone able to explain? Thanks alot in advance.

 

[evolutionZ]

Bump!

 

[Lostinthedark]

If your noticing algae it could be your water quality deteriorating. My first guess would have been high nitrates but I see you tested for that. I stopped using any "vitalizers" because they always made it a challenge to keep water quality high. When I don't know whats going on my cure is always a succession of water changes.
Good luck

 

[evolutionZ]

my nitrate have never went beyond 5ppm.. i make sure i check once a week and it was most of the time 2ppm. I use the zeovit vitalizer but not much difference noticed.. 2 drops for 50g but my tank's 65g so its kinda underdosing too..