it all depends on the size and shape of the tank, and the kinds of plants you use. i find it best to provide at least 1/3 of the tank surface covered in a "canopy" so they can escape the light when they want to, and then have a low-lying semi-sparse "yard" to swim through.
So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth,
And pray that there's intelligent life somewhere up in space,
'Cause there's bugger-all down here on Earth.
Loki/Grendel/Gail/Beowulf/Camilla/River/Bathory/Arwen/Lafayette/Deb/Jack Skellington (soon)