I'm gonna set up one of these under my 25H tank and I'm gonna try a no-filter reef tank with just circulation. Put in some live rock and live sand and see what grows. Gonna have 110w of PC lighting. Hope this turns out pretty cool.
if you have never tried, and like the looks of one, try a planted tank. theres plenty of small fish you could add to the planted tank to have something to be proud of
IMO its best to start out small with anything like a reef or planted tanks to get a feel for it. this way if you happen to melt your plants, you wont cry as bad over a 20g tank as you would after all the time and money invested in a larger set-up.