Aside from the test results, I note the following (recorded in an Excel spreadsheet):
- Water Temperature
- Top-offs
- Backwashes
- Any other chemicals that are added on an occasional basis (like when on vacation) such as trichlor pucks, Polyquat60 algaecide
- Temperature hi/lo for the week along with any notes on other weather anomalies that occurred that week or whether we had a pool party that week.
- Rainfall amounts for the week
The test results plus the above items are noted once a week. That's enough to give me sufficient trend info. Each year, I save off the prior year's log and start a fresh one for the new year. This way, I can compare trends across years.