I'm not yet using mine, but it will have the following fields:
- Date
- Ammonia
- Nitrite
- Nitrate
- Ph
- Kh
- Actions performed: I will write down filter mainteinance, water changes, fishes added, dead fish removed, plants, changes in lightning, etc...
Beside these values, you can also keep track of CO2 levels, O2 levels, phophates, etc... just add whatever you need or like to control.
You can also use an agenda and write down those values in the date you perform the checks.
My 2¢