Most of the things that would really make an interesting study wouldn't look all that pretty, may take significant time to establish, or would be best done with multiple near identical tanks. There are a couple things I can think of which would be useful both to the hobby as a whole and not require much skill going in, but may not look pretty. This probably isn't original research, but the right text around it could do some good:
Nitrate consumption of macro algae. Pick one, preferably something controllable like chaeto (pronounced Key-toe). Dosing controlled amounts of nitrates is easy, and with daily dosing you should be able to keep a steady level. See if X amount of chaeto (normalize consumption to algae mass) can consume more nitrates when it is more available, under different lighting situations (hours per day, intensity, or spectrum, pick one).
You could do the same with calcium consumption. This would be more fun with a coral, but would probably be easier with something like halimeda, although determining what algae mass is at work would be more interesting.
From a different route: How does flow (shape, intensity, or pattern) change the growth structure of something? This will probably be easiest with something that colonizes, so halimeda would work better than chaeto. It would be the most fun with an SPS or LPS, but would take a much longer time frame to control. You *might* be able to run three colonies of one species of a monti cap in a tank that size. Perhaps one right in front of a powerhead, one across the tank, and one in the center. Keeping lighting the same at all three, or having enough samples at each condition to draw meaningful results might be tricky. I'm picking monti caps because they have interesting flow dependent shapes, take shape pretty fast, and are pretty easy to grow. Other corals may work.
