Major rookie mistake need help

It was 7.8 at the fish store. I usually keep it around 8.2

 

mine sometimes drops to 7.9 i don't see 7.8 killing fish. you may want to look in other directions

 

I wouldn't think that would kill them either, but they were only in there a day, then I had the mishap with the salinity and then did a water change..basically I panicked and I think the fish paid the price. Although the tang seems to be doing well. Not sure if maybe it was a bad batch of fish... I don't know much about the six lines and the royal gramma whether they are delicate fish or not..definitely just starting out. Feels pretty crappy though.

 

come to think of it the 7.8 is a day time reading so your night reading would be lower than that.
still I'm guessing the fish were just stressed to start from just putting them in the tank. i didn't realize you just added them. sorry for your loss. i know how losing a fish or 2 feels

 

I have also made similar mistakes. When i ve needed to make adjustments, i have used mixed salt water to do my twice daily top off. It is usually a small amount so it does not cause a rapid change, just a slow steady increase until it got back to where i wanted. +1 on the fact that you have probably been low for a while since you just recalibrated.