Are they dead???

[baloutang]

I had a magnesium OD in my tank, yesterday. I did a 40% water change and my tank params are pretty normal. A few snails are belly up (not dead) and my 2 brittle stars are not moving, they didn't move that much before this happened.
How do you know that they are dead? Their color looks good as before, but I can't tell if their appendages on the underside are flowing only because of the water flow or not. I would also assume their bodies would be more turgid rather than squishy and soft.

[Reefmack]

Why do you think you had a magnesium OD? Salt mixes are typically low on Mg and I can't imagine a water change ever adding too much. Are you dosing with a Mg additive? I've dosed Mg in an attempt to kill off Bryopsis algae, up to in excess of 1600 pmm, without affecting any life in the tank. Did you add freshly made salt mix? It should be made up, aged a day or more, and then stirred and the temperature adjusted to tank temperature before adding. A 40% water change is pretty large - something may have drastically changed the water parameters if it affected the life in the tank. I'm just making some guesses.

[CadetMKultra]

Pick them up. They'll move off your hand with surprising speed, if they're ok.

[baloutang]

Yup, they died. I removed them from the tank immediately when I saw no further movement and put them in a 5 gal bucket with heater and airstone to see if they were truly dead. Next day the water was totally fouled which was a good indicator of their demise.