Fishes suddently dead

[hanbo01]

Hi guys.I'm now really confusing.I've runing my 100g reef tank for about 1 year.But 1 day,cothurnus happened.When I waked up and look at my tank,all of my Cleaner Shrimps were dead,as well as a lemonpeel anglefish,a potters anglefish and 3 Azure Damselfishes.On the day after,I found the flame anglefish was struggling with the kyte side up and operculum opened very widely.I took it out and put it into my other 20 liter tank,it became better a short time later( and it is still alive in my 20 L tank till now).
A few days later, another 2 Ocellaris Clownfishes was dead either.I've tested my tank with Salifert test kits.As the result,there is no NO2,NO3,NH4,abusolutely none of them at all.
The only thing seems not in good level I think is Mg. The Ca is 370 ppm but the Mg is over 1300ppm.Should that be the reason?
Also I got about 5CM sand in my tank with no sand cast at all.Is it probablly the H2S comes from the sand?
Any idea about it?Really need your help!

[francis1123]

did the fish show any sign of disease?
what was the last item added to your tank and when?
it really sounds like an ammonia issue to me. struggle breathing and then when you moved it to a different tank it quickly recovered.

[hanbo01]

Actully the last thing I've added to my tank was MgCl2 and NaHCO3.The thing makes my cofusing is,I use the water from the 100g tank to fully change the water of the 20L tank every 2 weeks before this happens.So the water in the 20L tank is just the water in the bigger tank in the condition of 2 weeks ago.

[francis1123]

by item added, i was asking last living item. fish, coral, rock.
if something in the large system died within the last 2 weeks it could cause an ammonia spike. it could even be something you didn't know was in the tank, hitchhiker. there are many factors that could be playing into this. i would personally do a series of 25-50% water changes every other day for a week or so. that should bring parameters back in check and help cycle out whatever is in the water.

[hanbo01]

I've still got 1 blue tang,1 Purple Tang,2 Percula Clownfishes and about 40 pieces of SPS.
I'll try to change the water in the way you suggested.Thanks a lot guys!