LEDs won't save you a lot in terms of electric consumption compared to fluorescents with electronic ballasts. (the old transformer ballasts waste a fair amount of energy).
However, for an aquarium they are significantly more efficient than fluorescents; all the light goes down, rather than rattling around in the reflector, which loses maybe 30% of the light, mostly to having it hit the fluorescent tube again.
LEDs are also smaller than a fluorescent system with the equivalent output, are very easy to adjust your color spectrum / PAR rating, and relatively easy to dim (fluorescents can't do that)