Simpte's twin fixture may not fit on to your existing hood so you would need a glass hood. This is another good option:
It is very narrow and almost as much wattage, but a better reflector. I have the 24" 28 watt version on my 20 gal
. I love it and my plants do too.
The other option is to overdrive your NO
bulb by buying a more powerful ballast and hooking that up to your existing hood. That gets rather complicated and best for those comfortable with wiring. It also generates a lot more heat.