I think Ali mentioned Helios fixtures. Upon further research it looks like they are phasing out their T5-II series, to be replaced by T5-III series (replacable bulbs, being one new feature).
I'd be interested in suggestions for options I might have in upgrading at least half my lighting, if not all of it.
As a reference, here is a picture of my existing lighting scheme. Note that two bulbs are covered with white reflectors, the other two lay over the filter compartment in the near left corner, just barely fit.
