I also think you'll be fine if you don't have a lot of surface agitation. -but this is something you might want to check at different water levels, if your tanks are ever neglected long enough to loose some volume.
If you are still concerned, I believe you can usually find waterproof end caps online. I've also heard of people using things like bicycle inner tubes, or plasti-dip (I think that involved directly soldering the leads to the bulbs, which would make changing them somewhat troublesome). If you are going to use T5, you could probably use the larger diameter shrink-wrap.
But, all that is probably overkill, I think these were mostly used on reef setups, where there was surface agitation, salt deposits, and concerns about the glass absorbing UV or something.