well - the password thing is bad - a new install would probably help to remove the password.
if you don't have the system disks - download them - they are free
more informations about this: [http://lowendmac.com/macos.shtml|click]
a fish tank is funny - but i would make a little upgrade - mini itx board (santa rosa), 2gb ram, a nice large hdd, wifi and a nice lcd and so on - there are many ways to build something like this - even a half toasted MacBook logic board would fit (had a few with burned away battery connectors - the are running but never would be able to run battery powered).