Lighting a 20 Hex

I have a 20 gal hex tank that currently has PC light of 28 watts. It's low tech & I'm satisfied with the results I see in there. The problem is that it seems to be getting harder to find bulbs & even when I order them online, they're hard to find in stock. I recently stocked up but the one that has been in use for less than a month, often does not come on unless I fiddle around with it & the connector. I'm thinking it will be time to move on when I use up this batch of bulbs, which apparently they are going quickly as the current bulb is already turning dark at the end.

Any suggestions that will fit in the small footprint of a 20 hex & give me comparable results? I don't think I've seen any T5s that small.