Softened water is safe for most pets to drink.
The only exceptions for softened water are some exotic, tropical or ornamental fish. We recommend getting advice from an aquaculturist if you’re unsure.
EU Directive on water quality:
Softened water meets the EU Directive on water quality, meaning it is safe for human consumption. This includes consumption by pets.
As with humans, softened water isn’t recommended for pets that have a restricted or sodium-free diet.