It certainly is confusing! Lucky for you a longer fixture of the very sort you are looking at is on the chart on this thread! See the fourth chart. A 36" long Odyssea T5HO puts out about 35 PAR at 25" and about 50 at 20" Be prepared to raise the fixture or put something to shade the tank like window screening or floating plants and you are good to go.
Lighting an Aquarium with PAR instead of Watts