Hi there!
30
Theoratically speaking if it's safe for you it's safe for the fish. If your filter is big enough, you don't have to worry because all the ammonia and chlorine gets broken down really quickly. For my 600l Aquarium, I change 30% of water using tap water and don't put dechlorinator. Fish fine after almost a year.
Cheers,
Happydanio
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