As Greg said keep a log/spreadsheet of your test values.
Down the road it will be invaluable because you will be able to see trends and see where you were at when things were good and where you were at when things were bad.
Also keep track of when you purchase corals, fish, who you bought them from, when you add or change equipment, change bulbs, change additives etc.
I have weekly log dating back 3 years to when I first started my tank. I can easily see the good and the bad and remember the disappointments. You can also easily measure coral growth. Pictures help too.
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