Im definately gonna use led's on my tank. There is a store here (NZ) that has a christmas tree setup with a control, for different light settings, and 90 blue leds, for about $28USD
Another thing you could use is those cathod lights that are for car dashboards,mini versions of the fluro things under boy racer cars (yuk)
They are pretty cheap and look real cool- on a fish tank, (not a car)
Since an octo doesnt care what light it gets aslong as its not too bright, LED's are perfect. They are cheap, compact, and WAY more efficient than a "proper" light.
Also you can get any colour you want.