Pour the cold filtered water into a bowl. Evenly sprinkle the unflavored gelatin over the water and let it sit undisturbed for 5 minutes to bloom.
Pour the boiling water over the bloomed gelatin. Whisk continuously until the gelatin is completely dissolved and the liquid is clear.
Stir in the fresh squeezed citrus juice. Add your choice of sweetener a few drops at a time, tasting until it reaches your desired sweetness.
Pour the mixture into individual ramekins or a glass dish. Refrigerate for at least 4 hours, or overnight, until completely firm.
Serve chilled, optionally garnished with fresh mint or berries.