Water Lettuce is TOXIC to Betta Fish.
Water Lettuce is a fast growing floating plant that is common in the fish keeping world. It is so fast growing that is banned in many states in all forms including AL, FL, CA, LA, MS, TX, and WI. This is less due to the toxicity and more due to the fact that this fast growing noxious weed can quickly overtake waterways and kill native plants. It will also cause oxygen depletion due to the thick surface cover it provides and starve the native fish of oxygen. Frankly it is our opinion the plant should be eliminated from the fish keeping hobby as it poses a real threat to native fauna and is easy to accidentally release during water changes.
But for Betta specifically this plant has calcium oxalate crystals located throughout the plant but most heavily in its thick roots which the Betta fish likes to nestle in. They are also prone to nibbling on them which can lead to illness and death. We have experienced this issue first hand and recommend against keeping these plants with your Betta.