oops! again, Yes davejnz, I did misread your post about the freshwater/fresh saltwater.
Chrisl1 - I usualy keep the water level down below the inner tank rim so I could add or loose a gallon.
I think I have a better handle on how the drop in SG came about. I switch salt products and had not paid attention to the mix, this was back when I was changing water more often.
To sum it up I caused the drop in SG by not paying attention.
Davejnz - eeww - I don't think I want to pour the skummy stuff in my hydromiter - let just leave that question a mistery