Peel wide strips of zest (avoiding the bitter white pith underneath) from half the grapefruit and the whole lime. Juice grapefruit and lime into a liquid measuring cup (add water to reach 1 cup if needed). Pour into a 6- to 8-qt. pot and add zest strips.
Cut off green tops from lemongrass (about half the stalk) and peel first two outer layers from stalks. Mash stalks with a meat mallet and cut into 2-in. pieces. Add to pot of juices along with sugar, honey, and 3 1/2 qts. water.
Heat citrus lemongrass mixture over medium-high heat, covered, until boiling; boil 1 minute, then remove from heat and let cool.
Meanwhile, mix up your sanitizing solution using SaniClean and water (follow instructions on SaniClean label). Sanitize all equipment except the capper; invert bottles into bowl to drain. Let everything air-dry.
Strain sweet citrus lemongrass water through a fine-mesh strainer into sanitized 1-gal. container. You should have about 2 in. headspace; if you don't, add lukewarm water until you do. Dissolve yeast in 1/4 cup lukewarm water and stir into container. Cover container with a double layer of cheesecloth, secure with a rubber band around the lid, and set in a warm place for 2 hours.
Stir mixture thoroughly, then use funnel to slowly pour soda into bottles, leaving 2 in. headspace. Cap bottles with capper and shake each bottle. Let bottles sit at room temperature for 1 to 3 days, opening a bottle each day until you like how it tastes and how much fizz it has. When you like it, chill the bottles.
Note: Nutritional analysis is per bottle.