Water does go bad eventually of course. But if you change it often, you don't need bleach. I have heard this too about adding bleach, but it will destroy good bacteria in the birds body and the birds may drink less water because of the taste. Water should be changed daily due to back wash from...
The problem with bleach is that it will destroy the good bacteria in the crop and the rest of the GI tract. The crop needs these good bacteria for good digestion and by destroying them, you are asking for sour crops. :-)
Yes, algae can be deadly. It was no coincidence. If you wish to add apple cider vinegar, mix it 1 tablespoon per gallon of water. Try to keep the water fonts out of the sun and they will not grow as much algae. :-)