Changing the bulb twice a day is going to get annoying very fast. You should be fine with 1 t8 bulb if you want to grow some mushrooms, leathers and other soft coral. Your corals will look brown with a normal day light bulb. You can test them out see how each looks and then make a decision.
If you want to stick with just one bulb, get a new bulb. Something like a 50/50, blue+ or whatever color bulb you want. You can also pick up a second fixture pretty cheap. Look on Craigslist or go to Home Depot and use a shop light.