Drinking tap water lowers your IQ because water companies add fluoride to your water; And the American Government has proved this in a 9-year study. It showed that for ever one part per million of fluoride in your urine, it lowers your IQ by one point. The study also shows that children exposed to the highest levels of fluoride had the highest drop in IQ.
The easiest way to around this is to start filtering your tap water. You want to get a filter that has been third party tested and shown to get out fluoride. I also recommend getting a filter that takes out heavy metals, feces, medications, hormones. All that stuff are shown to be in your tap water.
If you’re still drinking tap water and you’re not feeling smart about it, drink either glass bottle spring water or well-filtered water.