A spreadsheet can be useful for tracking changes and recording test results. A log can provide insight into how changes may have affected the system. Use a log to track changes in water chemistry, lighting, medication, filtration, current, fertilizer, filter cleaning, animal population, the addition of new rocks or other decors, and water changes.
Many changes hobbyists make to a system can take months before a trend is noticed. Reviewing a log can expose what changes made in the past may be showing results months later.