Mini Brittle Star Mass Die Off

[ScubaSteve]

So, weird thing is happening today. This morning I saw a mini brittle star lying wrong side up on the sand by a rock. Didn't think too much of it, sometimes they do weird things. Got home after work and there were a couple on the sand and a few on the rocks looking pretty lifeless. I pulled one of them out to check it out and at first it didn't move... but then it twitched just the end of one of its arms. Alive, but barely.

I am now looking in the tank late tonight and I can see at least a dozen of the little guys, some on the sand, some on the rocks, some half out of holes. It honestly looks like a battlefield of corpses. I don't know about you, but this worries me. I see this as a canary in the coal mine.

The only thing that I had done different this week is that I have been raising my mag slowly using magnesium sulphate. It was low last weekend at 1050 ppm and I've been slowly raising it over the past 5 days by no more than 80 ppm per day. I checked my salinity and found that it had gone up to 1.028 from 1.026 last weekend; I am attributing this raise in salinity to adding ~750g of magnesium sulphate (epsom salts) in order to raise the mag. I'll be lowering the salinity slowly over the next week or two. I added the mag slowly; added a bit on Sunday to see how things went and then slowly added more to get to 1400 ppm over the next few days.

The other strange thing that has happened recently is in my strawberry top snail. He is still alive, but like the brittle stars, barely. He's tucked unto his shell but not all the way; his foot is still sticking out. Sniff test and a poke to the foot tells me he's still alive, so he stays for now. But he otherwise looks pretty lifeless. I've seen him do this once before (so I'm not all that worried) and it was after a tank swap when the magnesium was also low. This was my first indicator that my mag was low. Test kit (Elos) confirmed this. Mag dosing commenced (slowly)

Parameters:
Alk: 9 dKH
Cal: 430 ppm
Mg: 1450 ppm
pH: 8.0
SG: 1.028

Ammonia: 0
Nitrite: 0
Nitrate: <5 ppm (it actually reads 0 ppm but I never trust it to ACTUALLY be 0 ppm)
Phosphate: <0.25 ppm (again, says 0 ppm but I don't trust it)

The tank was swapped 1.5 months ago to the new system. Something went weird in the transfer and it cycled. Hard. I lost all but a handful of my SPS (expensive frags went to a friend's tank). So I don't know if these weird happenings are part of the ensuing aftermath of the tank swap or if there is something seriously bad about to happen. I'm waiting for a small cycle if these brittle stars are dead.