There is no evidence that fluoridation harms dogs (or any other animals, for that matter). The US fluoridation standards set fluoride levels at 0.7 to 1.2 mg/L; natural groundwater levels of fluoride can be as high as 50 mg/L (levels where mineralization of bones may be affected). What this means is that tap water often has much safer levels of fluoride, because water authorities will remove fluoride if natural levels exceed the drinking water standards.
If you're concerned about contaminants in your drinking water, the first step is to check the Environmental Working Group's database. Most towns have safe drinking water, but some do have measurable levels of contaminants of concern. Check your own town here: http://www.ewg.org/tap-water/home
If you do live in a town with water quality issues, carbon filters are effective against most contaminants of concern. Regular old faucet-mounted filters are cheap ($20 to $30), readily available at big box stores, and they work if you maintain them.
Tap water is MUCH safer than most bottled water, domestic well water, or spring water.