All my fish are dead

I was trying to raise my Mg I mixed 1 1/4 pre made Mg(epson salt) into my sump. Could this have killed my fish? Ammonia 0 Nitrites 0 Nitrates 5 ph 8.4 Calcium 300 MG ?????

Looks like it did. 1 1/4 *what*? Milligrams- cups? Pounds?

Are you testing for Mg? Never ever add anything you have not tested for.

1 1/4 cup of liquid. I mixed 8 cups of dry epson salt with 1 gallon of ro water. I had added 1 gallon (1 drip per sec) of limewater. also 1 gallon of buffer(baking soda). The web site I got this mix from said you should add 1 1/4 cup mg after adding the limewater and buffer water. All my fish were alive until I added the 1 1/4 cup mg. I have a 125 gallon tank with a seaclone 150(modified for better skimming) a wet to dry filter, and 3 200gph power heads in the tank fo circulation. I have emptied 75 gallons from the tank is this enough to fix the water issue?

Are you sure that website recipe wasn't for pickled herring?

That's why I couldn't comment on what happened- I couldn't understand why so much of so many things was added all at once- without discussion of any parameters.

Wonder what website this came from?

My calcium was at 100 ppm. So I had been looking for a good kalkwasser to bring it up. I found a web site that list it was the same as the over the counter two parters. Well now after being upset at the loss of all my fish, I realized that I have to be able to test for a chemical before adding (Mg).

(Mg).

Calcium of 100 is pretty suspect- I would double triple verify that test kit. Any water made with any salt mix has to have more than that.

Have you been adding loads of buffer? What is your alkalinity? If the parameters are that far off I'd suggest lots of water changes to bring everything back into balance.