Britta themselves don't recommend using them without consulting an expert:
# Can I use filtered water for tropical fish?
Due to the sensitivity of many tropical fish to even the smallest changes in water quality or conditions, such as temperature and pH, as BRITA advises consumers not to use filtered water for tropical fish without consulting a specialist.
Once again I have no knowledge whether its safe to use, im just posting stuff I find, if I find a conclusive answer I will let you know.
yes i am aware of this. but if you read the comment you can see, they are just covering their backs. none of the things mentioned above, will have any more effect than if you used carbon in your filter. the only problem we as fishkeepers have with carbon, is chemical leaching. the same is true of a britta filter. if you look at the water recovered from a britta, you will find no silver, well any measurable amount anyway. and there is far more good stuff in britta water than you will find in RO water, which is potentially far more dangerous than any form of carbon filtering. requiring additions of salts minerals, and everything else the membrane removes, just to get back to where you were if you had used a Britta in the first place.
i repeat there is no potential of silver getting into the filtered water, which i believe was your original comment!
though expensive, water treated this way, especially when it is being added to a live tank, is perfectly safe. and vastly preferable to using RO and the water is not acidic, like RO. it even leaves the calcium in the water. its softening effect being restricted to removing lime deposits from the water.
i am not saying this is the best way to solve hardness problems. but it is better than RO. however provably incorrect comments like " silver in brittas is deadly to fish, must be refuted.
