Place 1 thinly sliced grapefruit 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 3 hours to allow the grapefruit to infuse. Stir well and strain, discarding the grapefruit. For serving, add fresh grapefruit slices for garnish and plenty of ice. The infused water will keep refrigerated for up to 2 days.