Look at a WordPress-based blog
I use "Private posts" on my WordPress-based blog to keep track of water changes and other fishy stuff (as well as watering and fertilization regimens for my orchids and carnivorous plants).
There's a mobile WordPress app for Android, iPhone, BlackBerry, etc., so I write the blog entry while I'm doing the water change or whatever. Takes maybe two minutes. Easy peesy lemon squeezie.
I use categories for each tank, as well as for each fish and aquatic plant species (I'm one of those GWAPA geeks), so I can fairly easily pull up a history of tank maintenance, fish breeding, or answer questions like "Where did I plant that Cryptocoryne balansae I got at the GWAPA meeting last month?" That kind of thing. Works for me, anyway.