Combine the raspberries, sweetener, and lime juice in a small blender.
Blend until smooth.
Add to a large glass and top with seltzer. Stir gently.
Add ice to fill the glass.
Notes
Sweetener - You can sweeten this however you like. A sugar-free sweetener, cane sugar, agave nectar, honey, etc.Simple Syrup - If you are using a granular sweetener, you can dissolve it in a bit of water first to make simple syrup.Notes on Sugar-Free Sweeteners: