how much light for a 5ft tank?

[Tristan]

hi guys, I haven't been keeping fish for really long and I'm not familiar with t5 tubes. does the 2-3 watts per gallon rule still apply? I plan to have a pretty heavily planted tank.

also, if i get the the ready made fixtures from DE lighting, can i put the fixture directly on top of the fishtank? they're all called 'retrofit' so i'm assuming i have to put the fixture in a casing or something.

edit: oops i should have posted this in the beginners forum.