it really does depend on the fish. The 1 inch per gallon is a good starting point, but the footprint also has a lot to do with it. Im guessing this is aprox 3' X 1.5'
Ballpark figure? 30 - 40 inches of fish, so maybe 15-20 Tetra / Guppy (not 15 each) and 5-7 or so corys?