Ekashna is the Jain practice of taking only one meal a day. Regarding brushing during ekashna:

• After you finish your one daily meal, it is appropriate to clean the mouth and remove food residues. This typically includes rinsing the mouth and brushing the teeth.
• Use a non-violent, Jain-friendly hygiene method. Many Jains use a soft-bristled toothbrush or a natural toothbrush (dant katha) and/or a miswak/datoon, and rinse thoroughly with water. Do not swallow toothpaste or rinsing water.
• The key principle is to maintain purity and cleanliness without causing harm to living beings. So, choose a toothpaste or cleaning method that aligns with ahimsa (non-violence) and avoid ingredients that would cause harm or require non-vegetarian processing.
• If you are unsure about the exact method your tradition follows (Digambar vs. Shwetambar), follow the guidance of your local guru or temple tradition. Differences may exist in accepted cleaning aids or preferred substances, but the aim remains same: mouth cleanliness after the sole daily meal.

If you’d like, I can point you to a JainKnowledge.com page that discusses ekashna and daily purity practices. You can read more here