Is filtered water (Brita filters, etc.) healthy for dogs? If so, what are the risks? I know that some owners give their dogs unfiltered water, e.g. from the sink. I was also going to give my dog some filtered water but wasn't sure if it was safe.
Depending on where you live, I don't know that I would consider it necessary to filter water for pets, however people have expressed concerns with respect to many municipal water supplies and their chemical treatment. The reason I mention this is that there have been concerns expressed about normal tap water for pets.
Now, with respect to filtering, there's no reason for this to pose any risk to your dog (or cat for that matter) and there's some opinion that it's better. We use Drinkwell Fountains for our cats (we're on a well system) and these are designed for both cats and dogs, by a Veterinarian, and would see no reason that they would need to be different in this respect. They use a charcoal based filter system very similar to what you'd find in a Brita system and there's no evidence I've seen that this poses a risk to either animal.
Now, this question can be answered logically, without knowing anything about pets, or if the water is for dogs, horses or rabbits.
Think about it! If the filtering is just this: filtering, i.e. removing stuff, chemicals or particles from water so as to make it more pure, how could this possibly pose a risk? This would mean that pure water poses a risk (which one?) and additives, like calcium compounds, chlorine compunds, dirt or bacteria reduce that risk.
I'm not sure about filtered water, but Smartwater and its derivatives should be a great viable alternative. The electrolytes in it will help to hydrate your pet.
Check out http://www.petmd.com/blogs/fullyvetted/2009/july/magic-waters-why-i-love-smart%C2%9D-water-therapy-pets-6860 for some awesome advice.