Dead Seahorse

So, something went terribly wrong with a water change and as a result one of my horses died. I don’t know if this has anything to do with them being under stress from whatever it is that we are treating, but:

1. I siphoned out 25% of tank water
2. I filled a trash can with 15 gallons of replacement water that I got from my LFS (same one I always used). The water was about 5 days old
3. I pumped the new water into the sump

Almost immediately one of the seahorse headed for the surface, looked like he was breathing with his head partially out of the water, sank to the bottom and laid there, eventually got up but never got over being “woozy.” He died about an hour later. All of the seahorses looked stunned and stayed hitched without swimming for some time, all but the one seem to have recovered. 

I did everything the same as I have always done it with the one exception that I did this water change in the middle of the afternoon instead of my usual 7:00 PM – 9:00 PM. Other than that everything else was identical (same equipment, vendor of saltwater, methodology, etc.). 

As soon as I saw that the horses were stressed, I tested the tank water and discovered that the salinity was high: it tested 1.026 – 1.027 which is higher than my usual 1.0245 – 1.025, and the PH was a lower than I would have liked (7.9) when my tank seems to normally be about 8.0 to 8.2.

For what it is worth, the cardinals and invertebrates (including a tuxedo urchin) seem to be fine — although I cannot definitively know that I have the right number of snails and hermit crabs as they are difficult to count.

Can you give me any insight as to what might have gone wrong? I desperately do not want to be hurting the animals in my care.