I almost feel like I cursed my octo sibling for posting how well he was doing a few days ago, but I lost Vader this afternoon and I'm not sure why. Yesterday, I did a routinue water change of approx 7 gal from my 75-gal tank. While this was going on, Vader was his usual curious self and kept "attacking" the python hose I used to siphon out the water. a few hours after filling the tank up; he became rather lethargic and did not want to eat (totally uncharacteristic of him). This morning I could tell he was stressed and displayed the same behavior my first octo did when I moved him from a 30-gal tank to the 75-gal tank and the water went thru an ammonia spike. Since I'm fairly confident a 10-gal water exchange should not produce an ammonia spike, I checked the salinity and measured it at approx. 1.026. I immediately siphoned out 10 gallons of water and lowered the salinity down to 1.024. I thought this had done the trick because he immediately started moving around and even ate some fresh shrimp from the feeding stick. Soon after though he started swimming back and forth across the tank repeatedly (which he never did before). This initially alarmed me, but then he settled down into the rocks and I thought all was well. A couple of hours later, I found him upside down all of his tentacles curled up and stiff and he had inked the tank. He was still alive for a few more minutes and then he was gone I am going to have my water checked by my LFS tomorrow, but the other inhabitants of the tank are doing fine. I am baffled as to how quickly he succumbed and can only think the high salinity was the reason. If anyone has any other thoughts on this, I'd appreciate the advice.