Volume calculator · Water change calculator
Free Printable Aquarium Water Parameter Log
Build a log sheet that has only the columns you actually use, then print it (Letter or A4) or save it as a PDF from your browser's print dialog. A freshwater fish tank rarely needs the same columns as a shrimp tank or a reef, so you choose. Nothing is stored or sent anywhere.
Tip: in the print dialog, turn off "Headers and footers" for a clean page. The page orientation is applied for you in most browsers; if not, set it in the dialog.
Which readings are worth logging?
The log is only useful if you test on a regular schedule and write the number down before you change anything. Many keepers record ammonia, nitrite and nitrate while a tank is cycling, then nitrate, temperature and the amount of water changed once it is stable. Shrimp keepers often add GH, KH and TDS, and reef keepers add salinity, calcium and magnesium. Your livestock's needs vary, so look up the ranges for your species from a reliable specialist source; this page does not tell you what values are right.
A useful habit: write the date, the reading and the action (for example "25% change") on the same line, so that later you can see which changes moved which numbers.