Short answer, sure.
I assume it's an 18 or 24" flourescent (the number on the end might be handy to bring with you (write it down, don't stick a piece of broken glass in your pocket))
It will often say something like F15T8 Which is a 15W T8 bulb or something similar. if you are unsure of the length, measure the space in the fixture and find one that measures the same.
AquaGlo are a nice bulb that are low cost.
120 Gal SW, 150lbs Prem. Fiji, 23Gal Sump, 2x250w 15K MH, 4x65w Actinic
3 x Halichoeres Chrysus (1 surfed), 3 x Nemateleotris magnifica, 1 x Centropyge bicolor, 11 x Scarlet Hermits, 6 x Zebra Hermits, 40 x Astrea Snails, 6 x Nassarius Snails, 3 x Cerith Snails