Who Gives A Fish Forums

If you're adding plants, keep an eye on lighting. Bettas don't need much, but plants do. 6–8 hours max, or you'll get algae. Also, floating plants can block light too much, so trim 'em if they take over.

Agree with hookit. Nitrites are toxic even at low levels. Your tank might be cycling or the cycle crashed. Monitor daily, water changes as needed. Also, check your tap water for nitrates—some sources have 'em high. If so, consider a nitrate remover or plants to help.