Originally Posted by Boredomb
They are both 18" bulbs
You might be able to manage, with the right tube, but if you don't mind upgrading the light, it would be advisable.
With the present fixture, you could use two Life-Glo tubes. These are made by Hagen, and they have two types, Life-Glo and Life-Glo 2. The Life-Glo (without the "2") has a coating on half the inside that directs more light out the other half, so you get slightly more intensity. I have one of these. Over a 55g, this could work for many plants such as those considered low to moderate light, but it would be pushing the envelope, so to speak.
A new fixture with 48-inch tubes would work better. This would spread the light full length, and allow you some options. I had a single T12 tube over my 55g years ago, and managed well with swords and crypts. The T8 tubes are better, more intense light and in better spectrum now from what was available back then (1980). You could go with a dual-tube T8 fixture. This allows you to have two different tubes, which is nice with respect to the colour rendition. A single-tube T5 would work, but not a dual-tube T5. T5 HO (high output) tubes are very bright by comparison, one is equal to 1.5 T8 tubes of the same type (spectrum) in intensity, and you really do not want more than two T8 tubes over a 55g for a low-tech or natural setup. The T5 NO (normal output) tubes are comparable to T8 but very hard to find.
I can suggest tubes if you ask.