There is definitely something off, with your set up and your time frames. You said you had the GSP for 3 months in this thread but posted on December 18th, less than a month ago, that you just got it. You also said in June your tank was two months old, which would make it about 8-9 months old now, but you said it's been up over a year. You also have other posts about struggling and losing fish not mentioned here, so I'm guessing they didn't make it. I'm not saying any of this is your fault, but accuracy is important when trying to troubleshoot issues.
There may be some sort of illness or parasite in your tank that has been there for awhile and is taking over or maybe there is something in your source water that is causing this. Are you using RODI? What are you testing the water with? Have you changed salt brands at all? Check for stray voltage? Do you have any sort of essential oil diffuser or air freshener in the house? There is a pattern here of losing fish, something has to be causing it.