There aren't very many ways for salinity to increase. Basically, either evaporation, unintended dosing, topping off with saltwater, or measurement error.
Given that one of your fish died, it's possible that there really was a rapid increase in salinity.
I'm not trying to be rude here, but the sequence of events isn't completely clear, so I'm going to rephrase it in my own words and please let me know if I have it right.
On Saturday, you cleaned the sand. Why did you clean the sand, and how did you do it?
After cleaning the sand, you performed a 50% water change.
At 6 PM, you checked the salinity and it measured as 35 (I assume you're using Parts Per Thousand and not Relative Specific Gravity).
After the water change, you noticed that one of your fish was distressed, and you did a 10% water change.
When you checked in the morning, one of your fish (the same one that was distressed?) was dead, and salinity measured 40 at that point.
Here I'm not entirely clear. You say you added 10% new water plus mixed in some osmosis water. Did you add extra salt water, or did you change 10% of the water and then mix in some osmosis to your tank to correct the salinity to 35?
After an hour, your salinity was once again at 40. In this same time frame, your pH dropped from 8.0 to 7.5.
How quickly did you mix up the saltwater? What's your procedure for making sure it's correct?
I think
@Seven Year Nap might be on to something. Assuming everything you've said is accurate and you didn't have a measurement error, the only way for salinity to increase would be for something to dissolve in your water. Could be undissolved salt, could be precipitate in the tank dissolving in the lower pH.
