A substitute for Eno fruit salt is baking powder. Add half a teaspoon of baking soda along with a teaspoon of lemon juice instead of Eno. You can also add a pinch of baking powder with it. If you don't want to add any soda, you will have to leave the batter overnight or for a couple of hours to ferment. It will not double up as a regular batter would, however, fermentation will still happen and the batter will be lighter. Temper the batter and then go on to make the idlis. This will however not be an Instant recipe and may not get the fluffy texture.