Instant Ocean killed my fish

[Bill]

So last night I did 20% water change using the usual RO/DI water but with a new 160g pail of Instant Ocean. This morning, everything gone!

Tank is 180gallon, the RO/DI membrane and cartridges are 3 months old. I also did 20% water change in my freshwater tank and all is good. I also mixed the salt prior to use in case of settling.

Tested saltwater shows no chlorine, ammonia, nitrites. Phosphate is 0.5ppm, nitrates are 5ppm. Alk at 3.5 meq/l, calcium at 380ppm. Temperature at 78F, sg at 1.024, ph at 8.3

I got the salt at edmonton big al's boxing day sale December 2014

I was wondering if anyone knows where I can get a full spectrum water test done for saltwater? Also considering necropsy for one of the fish.