Thanks for your question.
The brain is primarily fat. Since fat attracts toxins, anything which shouldn't be in the water we're consuming is likely going to end up in the tissues of our body. Fluoride, being a cumulative toxin (and rated as worse than lead), could easily be linked to the tumors.
Our bodies are 72% water. We should be drinking 1/2 our body weight in lbs in oz of water each day. So the importance of our water is magnified by the sheer quantity with which it's consumed. Filtering you drinking water (and your bathing water since anything on your skin is potentially in your body) is essential. 3 things you want to a filter to eliminate:
--VOC's (volatile organic compounds like "medicinal" drugs)
The one I use is one my website (www.triumphtraining.com) under the Andrew Recommends page. Called Aquasana, it's the best one I've found and the one I use myself. Whichever you decide on, realize that it's truly an investment in your health. And as you improve your own health, you improve the health of those around you.