Is it a planted tank (real or artificial)? If so, one thing that always makes a tank look bigger to me is the way the plants are arranged. if you put the shorter plants in the front of the tank and taller ones in the back, it can look really nice. Try to arrange your hardscape (rocks,
driftwood, etc.) toward the middle of the tank... not the center, I mean the middle front-to-back. I try and use the rule of thirds as far as placement side to side.
*edit* here are a couple of 55 gal aquascapes I've come up with in the past (although I no longer keep a 55). It's a difficult shape to aquascape as compared with a 75, which is wider front to back. Hope this is helpful.
