With all due respect, Brita tests its products for human consumption, not for amphibians running around in it. In all likelihood it is fine for most amphibian purposes. However, I do not use water sources for sensitive amphibians like dart frogs unless I can guarantee consistent water chemistry. That's why I always start from distilled and add chemicals to it myself.

It simply isn't possible to guarantee consistent water quality using a small filtration system like a Brita. I don't speak from an amateur or advanced amateur point of view - I speak as a Ph.D. chemist whose research for almost a decade was concerned with water soluble substances, pollutants, and their removal.