Assuming you are referring to Daphnia pulex, a common type of water flea, as food for bettas, the answer is it depends. While Daphnia can live in a variety of conditions, they generally do best in cooler water with little to no currents. This means that if you live in a warmer climate or your home is particularly warm, you may need to give them a break from time to time so they don't overheat and die. Additionally, Daphnia reproduce rapidly so if you are feeding them to your betta on a regular basis, you may want to culture your own supply to avoid depleting the local population.