Tank crash!

[FragIt Dan]

Alkalinity might have gotten too low, causing a large swing in pH and precipitation of Ca. I would take a water sample before doing anymore water changes so that you can test it later and hopefully figure out what it was. Hard to say right now with the limited info. How often do you do WCs? Do you dose? Do you use RODI? Do you let WC water stand for 24hrs before adding to your tank? Do you have corals? The fact that fish are dying is a pretty serious indication things are way out of whack. IMO, Mg would be low on my list of things to look at. I have dosed as much as 700ppm at one time and had it as high as about 1800ppm, unless you aren't doing WCs and it was really, REALLY low. Just a start to looking for the cause. Good luck and I'll keep an eye on the thread. I just about lost my DT before Xmas... let my Ca creep up over 600ppm! Did 2 50% WCs two days on a row and managed to recover. Burned and bleached out a lot, but it is all mostly recovered. Best of luck with things,
Dan