Seems to be an open source online database, I did find this but it's $24 - the demo only allows 2 items per catagory, it does have a nice little option to enter a common name the "search" for the specific that then fills in all the fields. Also you drag photos or click and it will do a google images search.
I've installed it and have it running (very little data at the moment) up on my web site (linux host). I'm really not much of a PHP/SQL person at all but I managed to get it working after getting over some stupid mistakes on my part. You can see it at http://www.lamantech.com/TankLogger/ (I threw in some sample data - but it's really sparce at the moment. I think I'll start using it for real though)
Mac Users : Accessing it is completely web driven. So it might just be a matter fo finding a host. But I'm curious and also downloaded tanklogger to my Mac and will experiment a bit. Just haven't tried to install it locally, but in theory it ought to work fine under MacOSX since it's FreeBSD unix. If you have loaded the developer tools from the OSX Install CD then you may already have apache, PHP, and mysql installed and not know it. I'll try to poke at it a bit to see if I can get a local version running on a Mac. More for curiosity sake.
My external hosting provider costs all of $15/year (http://jv-hosting.net/plan_a.php) so to me it's worth it just to keep it up there.
ON the other hand.... that Macquarium software does look pretty slick too! Will check that out. TankLogger is a definitely rougher around the edges but it's main advantage is getting at it from the web.