Its not common for fish to die when using elevated magnesium to treat bryopsis. I strongly doubt it was magnesium.

I figure your problem is either in oxygenation, or water chemistry. Can you post your full parameters? Ammonia, nitrate, nitrite, Ca, dKH, Mg, etc. please?

I thought plecos are brakish fish I'll have to double check that

Plecos are freshwater, Also I see in your signature that you are using 3 fluval 404's, what media do you have in them???I have found that canister filters to be Nitrate factories unless you change half the media at least once a week....Also I see that you had a lot of fish in your system which would result in a lot of organic material in your water column and when your skimmer stopped running you could have possibly had to many toxins in the water!!

When you pulled the dead fish out of the water where their mouths wide open, cause thats a sign that they died from not enough oxygen!

No it's not, but it is common for fish to die when the magnesium level has a big sudden swing.

sorry for you're losses, but what are you're water parameters corrently? such as nitrate,nitrite, ammonia? also if you were dealing with that magnitude of a microalgae outbreak, I'd imagine you're dissolved oxygen level to be quite low, from all the activity of the microalgae.

To me, this story sounds a lot similar to the demise of Steve Weast's Tank in Oregon. In his case, his skimmer pumped out too much water. No problem...but he needed to replace a lot of water in a hurry. Because he couldn't make that much RODI water, he decided to replace water using some dechlorinator that he had kicking around...and we've all heard the result.

My conclusion...Either the dechlorinater didn't work...or became a killer in of itself. I think that is your culprit...Chlorine or bad dechlorinator. Don't forget, Chlorine is absolutely deadly...that is why they put it in there.

the first thing that caught my eye aside from the microalgae was using aquaplus, isn't that aloe based? could've been a crappy dechlorinator like untame mentioned, just my 2 cents tho

I have 3 fluval 404's with the sponges and those cylindical filter media, other than that I have a phosphate media bag in one 404 and carbon in the other The third one has the fine mesh polishing filter.

Interesting point of the bioload and toxins. when my coral died, I left it inthere four a couple days to see if it would recover. I wonder if that may have caused it but then again I only lost them after the water change which would be odd. Good point of the fish well the sail fin had its mouth wide open but I didn't notice it on the other fishes. Maybe I was still in shock about the sailfin tang. I'm going to check the water parameters again to see whats going on.

Wow interesting insight I think we might have something here. I usually make my water reaplacement the day before but when I changed it last night I made the batch of water and put it in right away. I think I was pretty careful though b/c I mixed the water for a good 5 minutes and started out with slightly warmer water (to catalize the mixing) before cooling it down to 78 and into the tank. I've never checked if the aquaplus has to be refrigerated. Maybe it went bad!?!?

Ok just finshed the tests. My Calcium level seems really high for some reason.
If I remember correctly it is a side affect of the magnesium treatment.
I've obviously missed something else rotting the tank. Probably the one shrimp that I couldn't find and assumed dead. Should I do another water change? Man this is all too discouraging at this point in time :(

Salinity 1.024
PH 8.4
Nitrite 0.1 ppm
Nitrate 5 ppm
Ammonia 0.3 ppm
Phosphate 0
Calcium 680 ppm ??
KH 250 ppm