Place 1 thinly sliced orange and the coarsely chopped stalks and fronds from 1 bulb fennel in a pitcher. For still infused water, add 2 quarts of water. For sparkling water, add 1 quart of seltzer during prep and a second quart just before serving. Refrigerate 2 to 4 hours to allow the ingredients to infuse. Stir well and strain, discarding the solids. For serving, add fresh fennel, fronds and orange slices for garnish and plenty of ice. The infused water will keep refrigerated for up to 2 days.