All salt water fish in tank died at once

Question
All my salt water fish died at once this morning..We have a 30gal tank..
We checked all levels the day before they were fine.. We did a water change and 3hrs later everyone died (2 cleaner shrimp,2 percula clowns,1 sixlined wrasse 2 snails)and all levels spiked, salinity dropped along with the pH

Answer
Hi Jessica,
Im sorry, thats a horrible thing to find. If water quality was good, as you say it was, the day before, I would check the source for the water change, a quick death for all inhabitants like that indicates a toxin. Was it RO/Ri water or tap water? Test your source for trace metals and any other toxins(ammonia, nitrite, nitrate, etc). A bad source can crash a tank very quickly. The levels spiking could also be a result of the death of your pets. Decay starts rapidly in marine environments, so the levels you read are probably from decomposition. Again, Im sorry. Start with testing the source of the water for your change, and move on to any other possibilities. Bucket, hose, equipment, etc. If you have any more questions do let me know.