Either type of notebook should be fine; when the spiral bound or journal style book fills up just get another one.
You could simply have 2-3 columns:
Date is self explanatory.
Time is optional but it was helpful when going through cycling or dosing meds.
Event is basically what you observed or did. This could include anything from test parameters, changes in stock, observations, questions, etc. I test the tap water from time to time and this can be helpful in case params start to drift in your tank. Also, I am supposed to add root tabs at 30 or 45 days (depending on the type) and I can never remember what I did last week. Noting how many and of what type added and observing plant growth can be helpful in fine tuning requirements.
I record all of my notes in an Excel spreadsheet on a pc
. I monitor only two tanks and highlight the entry based on which tank it pertains to.