I don't keep a diary so much as records... I have 24 tanks and a hectic life and it's really easy to forget where I stuck that new platy I bought last week or when the betta fry hatched or if I fed the corys or whether I did the water change on the gudgeon's tank...
So what I have is a fish index. It's got little drawings on it that represent all the tanks I have. Each drawing has a tank number, and includes a list of all the tank's occupants, planting scheme, what equipment is in there and the tank's baseline water chemistry. I then have a daily feeding schedule (one sheet for each of the seven days listing what I need to feed all the fish, with room for AM and PM feeding) which I tick off on when I feed each tank. On top of that, a weekly maintenance schedule, write the date and tick off in pencil. So the date of last water change is always written there. I've got certain tanks that are to be done on each day of the week (3 betta tanks daily on weekdays and all the others over the weekend.)
When I'm doing something that requires detailed note taking like a betta spawn, I use separate records.
I'm going to be uploading most of the documents i use onto my website when I finish it... the html is giving me a few headaches at the moment. The layout works, but the links don't. I know what I've done wrong, it's just a matter of going in there and changing them all, and there are over forty pages that need to be fixed
burst into tears the other day when at the bottom of a page was a link to the day i brought jambo home, but loved seeing him 
