The water you should not use in a Keurig is distilled water.
The reason why you should not use distilled water in a Keurig is because distilled water is such a good solvent, it causes the coffee grounds to swell, and slow the flow of water which also leads to bitter, over-extracted coffee or possible brew basket overflow.
It is OK to leave water in a Keurig all the time as long as the water is refreshed daily as stale water can lead to mold growth and bacteria.
As long as you constantly refresh the water daily then it's fine to leave the water in your Keurig machine all the time.
You can use tap water in a Keurig.
However filtered water or bottled water is better in a Keurig to make the best coffee.
Tap water contains chlorine which can make the coffee taste bad for some people and filtered water or bottled water makes the coffee from Keurig taste better.
But tap water in a Keurig is just fine.
Any water can be used in a Keurig, but the K-Supreme Plus Smart coffee maker does not perform at its best with distilled water or high-alkalinity water.
While any type of water can be used in a Keurig coffee machine Keurig recommends using bottled or filtered water in your Keurig coffee maker.
Coffee and tea are over 98% water so using quality drinking water is necessary for a good coffee, tea or iced beverage.
All Keurig coffee machines and brewers can function properly with tap water.
Distilled water is not recommended to be used in Keurig machines because distilled water is such a good solvent, it causes the coffee grounds to swell, and slow the flow of water.
This leads to bitter, over-extracted coffee or possible brew basket overflow.