Certain ballasts only support certain types of lights. Standard
fluorescent will let you power standard fluorescent lights (like the
kind you use in shop lights, shop lights are what I use by and large
but I can only support low light plants, the light itself is bright
enough otherwise for sure.). Then theres very high output (vho)
fluorescent lights and ballasts for more light. Then there is compact
fluorescent probably the most efficient and bright, at least for the
amount of electricity they consume. There are other types of lights
which are much more expensive such as metal halide and others which
also use ballasts. In fluorescent the sizes and wattages vary and you
need to select bulbs for your ballast accordingly. My silver dollar
tank uses a shop light with t12 (one inch diameter fluorescent bulbs I
think and another type is t8 which I think is 1/2 inch diameter
fluorescent bulbs, there may be others) 48 inch 25 or 40 watt
fluorescent bulbs and it works nicely. Sometimes with cheap fluorescent
lights you need to buy and install a automatic fluorescent light
starter (a small tin can which I think contains what is called a
condenser) if you want your aquarium lights to come on automatically
with a timer, I installed a starter into my bedroom 15 gallon tank's 15
watt fluorescent light (accompanied by a 40 watt automatic screw in
fluorescent) and it was easy. Timers rule! Hth