DIAGNOSIS: Biofilm or mineral scale is building up in the internal reservoir and lines, which is common with any appliance that holds water at room temp.
FIX: For a deep clean, unplug the unit. Remove the filter. Mix 1 tablespoon of unscented bleach with 1 gallon of water. Fill the external reservoir with this solution and let it sit on the machine for 30 minutes. Then, run several hot water cycles through the system into a sink. Follow with several cycles of fresh water to rinse.
IF THAT DIDN'T WORK: The taste might be from the silicone tubing. You can try a baking soda rinse: fill the reservoir with a solution of 1 teaspoon baking soda per gallon of water, run a few hot cycles, then flush with fresh water.
HEADS UP: Never use bleach at full strength or leave it in the system overnight. It can damage seals. And always remove the filter before a bleach clean—it will ruin the carbon filter media.