You could also go with a 24" 150watt HQI setup. Or if that is too much, you could go with a 70 watt HQI metal halide. Those are great bulbs for freshwater, if you get the right Kelvin rating.
-Jon
everything they are talking about will need a whole new setup. i dont think you can get a higher wattage for a 2 foot fixture. you can try a bulb with a different color temperature otherwise you can add another light fixture beind the one you already have.