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The only parameter which will be different is pH. Typically your pH will be higher during the day and drop when the lights are out. Every other parameter will be the same no matter when you test.
I agree with the statements above except if you are keeping a log of your parameters. Then I believe it is best to take it at about the same time each day. That way your water temp. will be ~the same and you will be able to compare your readings more accurately.