I've used these: https://www.amazon.com/dp/B002HWRY5S/ref=cm_sw_r_cp_awdb_t1_CmXuDbBTBJ40E while on keto before. It's 10 calories, but that shouldn't end your fast by any means. It also doesn't have sodium, so you'd need to add some salt. I wouldn't pay more than $15 for any electrolyte mix.