04-28-2011, 12:07 PM
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Fluorescent tubes all come in standard wattage for the tube length, e.g., a 48-inch tube is 40w, a 30-inch tube is 25w, etc. Now there are some manufacturers making tubes with less wattage, for instance ZooMed make 48-inch in 32w, and they have the same "intensity" as the 40w of the same type; this is just a case of more energy-efficient technology. The point remains the same, the length of tube will be whatever watts, so you can't get more intense tubes than the standard, although the way they are manufactured may make some difference.
The reflector can be improved to direct more light into the aquarium. Tubes emit light all around, so about half of it is going to be directed up and around rather than into the tank. Some have had success by coating the inside of the fixture with foil, the shiniest side up. Or painting the reflector with some sort of reflective surface.
The other option is to buy a new fixture that holds two tubes full length across the tank length. Several manufacturers make fixtures that sit on the tank frame. You also need a plain glass cover between the water and the fixture; these are not expensive, but the fixture obviously is.
Two 18-inch tubes over a 60g is cutting it very fine. I'm assuming the tank is 4 feet in length. I think moderate light plants such as swords will need more light. But a photo of one would help, just in case.