If I could find a single-LED type mogul base lightbulb in the 20w range I'd jump on it. I don't seem to have any luck finding them though. Apparently there isn't much of a market for them (yet). If anyone spots something like this, let me know.
Have you looked at the PAR38 LED bulbs? They come with a standard base. They tend not to be single LEDs but keep in mind that neither are these floodlights, they are arrays of 1w diodes. Here's one Lowes carries in 18w - http://www.lowes.com/pd_338925-149-S...cagpspn=pla%22
And one specifically for aquariums at 21w - http://www.drsfostersmith.com/produc...09&cagpspn=pla