If they are the 2 pin flourescent bulbs you cannot use a T5 in a T8 ballast (AFAIK
)so you will need to change things around to make it work. As I said though, that is something you will need one of your parents to do. Safety First!!
With regular NO
bulbs on a 20 gal
tank though, the coralline will most likely suffer. What I would suggest is going to the LFS
and try to find some base rock. Some LFS
's will have rock that has been without light for sometime and little if any life on it. Generally you can sometimes pick this up at a decent price and still benefit from the fauna living within the rock. That kind of rock will give you all the filtration of the other rock you hear about without the need to worry about lighting...