I don't think the brightness is the problem. I don't think the rummynose tetras like the cramped quarters. They are one of the more active swimming tetras. I have a school in my 75g and they are almost constantly swimming back and forth the length of the tank. Besides congo tetras, the rummynose are the most active tetras I have kept. They are much more active than the neons, cardinals, emperors, blue emperors, blue tetras, black neons, green fires, and glowlights I've kept.
This is a Fluval nano, right? Looks like it with that background. I have an Ebi. I have a couple of dwarf gouramis in there and a couple of threadfin rainbows. Personally, as far as tetras go, I would only stick to the smallest, least active ones, like neons for a tank this size.