Always give your cockatiel clean, fresh water every day and keep their water dishes clean all the time, it's recommended you change a cockatiel's water twice a day, one in the morning and one in the early evening. Filtered water will be best.
Cockatiels can drink about half to one teaspoon of water per day. Environmental temperature, stress, frequent flying, or diarrhea will increase your cockatiel's need for water. Salty foods can make your cockatiel drink more water as well.
"Cockatiels will consume more water if they are feeding their young. Some medications, like antibiotics, may cause your cockatiel drink more water."
Drinking too much water for a cockatiel can be an indication for other illnesses like infections, kidney problems and even avian diabetes.
A cockatiel that looks good and happy and has no signs of illness should be OK and you should not worry about.
If you believe your cockatiel might be sick or if you think that your tiel is just drinking too much water, see an avian veterinarian.
Please do check the temperature of the room where your cockatiel is or direct exposure to the sun, this could create excessive heat on your cockatiel and of course more water consumption.
As always the information offered here is to provide guidance and is not intended to be a substitute for the good advice provided by your own avian vet. When in doubt always consult your own veterinarian.