The 1.5 - 2.0 watts/gallon guideline assumes fluorescent lamps, which are about four to five times as efficient at converting electrical power into light power. If you decide to stay with incandescent, you'll need 6-8 W/gal to meet those light requirements.
Is your light fixture specifically for aquaria? It would be unusual to see an aquarium hood with a screw-in socket for as high as 60W incandescent. If it's not an aquarium-specific fixture, you should be able to buy a standard hardware store/discount store household compact fluorescent (CLF) (the "swirley bulb") and screw it right in. Note that the rating on the fixture (60W) is electrical power. A 17W CFL may say on the package, "equivalent to an incandescent 75W" but it's OK to use in your fixture because its electrical draw is only 17W. In fact, if it fits, you can go with one that eis quivalent to a 100W incandescent - I think that's the largest they commonly come in.