Reusing fritter batter the next day is generally not recommended. The batter's leavening agents lose effectiveness, and the flavor can deteriorate.
Fritter batter typically contains ingredients like baking powder or baking soda to help it rise and create a light, airy texture when fried. These leavening agents are most effective when freshly mixed. Over time, they lose their potency, resulting in fritters that are dense and flat. Furthermore, the batter can absorb flavors from the refrigerator, negatively impacting the taste of your fritters. The longer the batter sits, the more likely it is to develop an undesirable flavor profile. While it might seem economical to save leftover batter, the resulting fritters will likely be of lower quality. It's best to make only as much batter as you need for a single batch of fritters.
If you absolutely must store fritter batter, keep it in an airtight container in the refrigerator and use it within 12 hours. Add a tiny pinch more of the leavening agent (baking powder or baking soda) just before frying to help compensate for any loss of potency, but be aware that the results may still not be as good as freshly made batter.