Systems that mount under the sink require a bit more installation and take up more space than faucet filters. They store filtered water in a tank, but can provide more heavy-duty filtration. The ROES-75 removes more than 1,000 different contaminants including 99 percent of lead, heavy metals, bacteria and viruses, along with chlorine and fluoride. Plus, it softens hard water without taking up a lot of space (its dimensions are 16 x 5.2 x 17.5 inches). Being a reverse osmosis (RO) system, this filter does produce wastewater—around 3 gallons per every gallon of purified water, which is on the lower end but can also vary—so don’t be surprised when your household water consumption spikes a bit. On the other hand, while some models tell you to run the faucet for 24 hours after installation (and then repeat every time you change the filter) to clear the filter of any loose filtration compounds.