When it comes to pretty much any maintenance activity that involves my tanks, I always take notes. Whether it's adding/removing fish, testing the water parameters, doing water changes, or anything besides feeding the fish, I always write down what I did. Why? Many reasons.
The day will come when something will go wrong in one of your tanks - water problems, livestock issues, etc. Rather than trying to rely on memory as to what you might have done or not done to contribute to the problem, often times a simple check of your notes will reveal the issue.
Do you remember how many of those black skirt tetras you added as dither fish three months ago? Do you do a dfc (dead fish check) nightly, such that you'll quickly recognize one is missing before his carcass becomes so decomposed that he blends in nicely with the substrate?
Or how about noticing that your pH has been slowly rising with each water change and subsequent test?
There are lots of reasons to take good notes about your maintenance activities. Use whatever method works best for you. I've used a pencil and little notebook (or several of them) over the years. Now I use a nifty little app on my Macbook called Evernote. Though not specifically for logging aquarium data, it's a great tool for keeping, storing, and searching notes of all types. Trust me, over time you will come to realize the value in keeping notes on your aquarium activities.
The Cichlid Room Companion
Tropical Fish Hobbyist
American Cichlid Assoc.
African Cichlid Hub