To keep bird bath water clean and fresh, clean hanging feeders regularly, remove feed from the area when sick birds visit, store seeds correctly, and scrub the bath itself. Other measures include removing dirty water from the bird bath and pressure cleaning it with a hose.
Dirty birdseed increases the risk of contaminated water. As such, it's necessary to clean hanging feeders on a bi-weekly basis. In addition, store the seeds in a clean, dry container to reduce the risk of mold, which in turn keeps the water clean. If seed becomes damp, throw it away. When it's raining, leave seed out for a few hours only to reduce the risk of mold. If sick birds visit, remove seed from the area for a week to discourage them from returning. When water does become dirty, remove it, thoroughly scrub the bath, and add fresh water.
Alternatively, remove the dirty water, and then power wash the bath with a strong hose. Following this, refill the birdbath with water, add 1 1/2 cups of bleach, and leave it to soak for 10 to 15 minutes. Thoroughly rinse the bath, and place a trash bag over the bleached water to prevent birds from landing on it and drinking it.