I used to have heartburn way back when. I took apple cider vinegar and that eliminated it ~10 yrs ago.
Tums only makes it worse. Heartburn is usually not due to too much acid, its due to too little acid and the sphincter muscle at the bottom of the throat getting lazy and inflamed. ACV will cover the acid part till the muscle can recover and close properly.
I used to take ACV on my Water Fasts along with vitamins and what not, but now a days I try not to unless I absolutely need it. Which I have not, but I've not done longer than 4 day fasts in a loooooooong time.