DIY Lighting - Florescent Lighting Confuses Me

[DGalt]

So I'm trying to put together a new lighting system for my 15 gallon planted and am having a hell of a time doing it.

The housing for the fixture I can figure out, same with the reflectors (although, if anyone has any suggestions about making reflectors I'd appreciate that :thm

Florescent lighting, however, is just confusing. All I want is something that I can put 2x 15W T8 bulbs (these bulbs http://www.lightbulbsdirect.com/page/001/PROD/Aquarium/F15T8-AR-FS) in to. Really shouldn't be all that complex right?

My real question is what is the rule about putting different wattage bulbs into a florescent fixture. What I mean is, if a fixture is fitted with a 20W bulb, can you replace it with a 15W bulb (I was thinking of using two of these: http://www.amazon.com/LIGHTS-AMERICA-7020-6-Light-Stick/dp/B00004WA4D)

 

[sandman26]

the rule is based on the balast that you have. On the balast it will say the max wattage of bulb that the manufacture says is apropriate. usually going with a less watts is ok and recomened.

 

[DGalt]

so what should I look for (b/c now that I'm thinking about it those pieces are kind of bulky). can I buy a ballast that will support 2 x 15W bulbs and then just get 2 T8 sockets? Are the ballasts specific to the type of bulb (i.e. T8, T12, etc)?