I keep a spiral binder, plain lined paper, for a fish log. With 20 tanks, a busy breeding & sales operation, water changes & other tank maintenance, and auctions & such on the weekend there is no way I could keep it all straight in my head. I refer to all my tanks by a number, 10 gallons are 10-1, 10-2, 29's are 29-1, 29-2, and so on for as many of that size tank I have running.
So, if I have a slow period, am low on fish & need to shut down a tank, I write the tank in the left margin, with the date, for example, 4/16/07, 20-6, then to the right I write in "Transfer 12 gold & 3 black angels to 29-5, drain, rinse, & store tank". I then write 29-5 in the margin, and to the right what I did with that tank.
Whatever I do with a tank I write the date along with the tank number in the margin, and what I did with that tank, whether it be a water change, switching of fish, adding or selling fish, fish purchased, whatever. This is the only way I can keep any of it straight. I number the tanks with a sharpie, and will often write the tank temperature, along with spawn & hatch dates on it.
Using a notebook allows me to use as little, or as much space as I need. A slow night like tonight, with no water changes, will only take a few lines. A busy weekend will take up a page. A planner doesn't have the room for a busy weekend.