I had a similar dilemma. I ended up going with the finnex 24'' Ray2 fixture (day/day) and hanging it about 2 inches above my tank. Including my substrate it's about 18'' from light to floor. I don't remember exactly but it's around 50 PAR for that depth. Raising it a bit helped to make up for the 3'' gap on each side and I don't notice any dark spots or plants leaning towards the middle.