Excel spreadsheet... if you're a spreadsheet type of person! Dates in the rows, water parameters and notes in the columns.
Within one spreadsheet, I have a tab/worksheet for my main tank, quarantine tank, as well as a saltwater "inventory" tab. Whenever I make an addition/subtraction from my saltwater mix/inventory tank I note it. Makes it nice to know exactly how much water is in there when it comes time to adjust the SG
after adding some too salty/not salty enough new water. I've got another tab/worksheet in there that gives me the adjusted salt to water ratios (for a higher SG
than on the package) for different quantities of water.
I also just added a couple columns that I "fill" with color to note when I added/changed carbon, or when I did a water change. Blue for water change, black for carbon.
Yeah - I'm anal.