I love Sonic... But this location has an issue with their slushies. I visited a few months ago and was disappointed when the slush tasted off - like their water filter needed to be replaced perhaps. It tasted like our sink water more than anything else. They said they fixed the issue and we returned - and it was much better.
Visited today and experienced the same issue - slush tasted off. This leads me to conclude it's an ongoing issue and I'm hesitant to order another slushie or water based drink from them again. If it's the water filter, (or whatever it is) they need to keep on top of the maintenance.