I keep one of those detailed journals on the computer since I have many tanks and something new is always going on. My categories consist of a spawn record data base, health on each tank, behavioral journal, general journal, water testing, water changes, etc. I also have a sales, purchase, growth records, utilities and food record journal. Yours needn't be so complex. Just jot down on a piece of paper with the date every time something happens that you think will someday be an important reference.
Journals can save a lot of heartache.
Well, I can point out for you why you would benefit greatly from keeping one. Mr. Fishy one day was acting especially dominant. He wasn't allowing any of his friends near the food. This is unlike him. So you jot down how he acted.