Mixt is pretty good and cheaper than GFuel, but adding it to carbonated water throws off the taste a bit, the CO2 makes it more acidic. I'll still add it every once and a while though to Sodastream

https://www.amazon.com/MIXT-Energy-Formula-Concentration-Starfruit/dp/B071FGHGGN/ref=sr_1_2?dchild=1&keywords=mixt&qid=1619478438&sr=8-2