Never had any issues with smell or slime or any kind of silt formation or any off tastes. My Zero sits on the counter out of the sun filled but sometimes unused for a week (I have 2). I have a glass gallon jug in the kitchen closet that I fill with Zero Water and my sodium bicarbonate concentrate. At that point I have both Zeros going, to fill the glass jug. I do thoroughly clean all jugs once a month.
I use that jug to fill my Decent water tank, which is an open ceramic tray at the base of the machine. I do notice that this tray can become very slightly slimy to the touch, if I don’t pour espresso for a week. I do Irrespectively clean that tray every week without fail, particularly as it is somewhat exposed to the environment and espresso machine piping.
Decent is in the process of perfecting a stainless steel tray cover that I will buy as soon as it is available; the cover is designed to allow filling without removal. Ofcourse, I will end up cleaning both once a week :). But some folks have found the odd mosquito or other floaty in their water tray and have requested the cover. Though when I go camping there are inevitably floaties and thingies in everything consumed and I am still alive.
When I am away from home for an extended period I leave the filled jug in the fridge, to keep the filter moist and less prone to bacteria development. Upon return I dispose of the water and clean the jugs. Some people go further and remove the filter from the jug and wrap it with plastic wrap and then place in the fridge.
Fridges do inhibit bacterial formation but there are lots of both bacteria and odor sources in fridges, including meats and cheeses. These can percolate into water. I have noticed that water that sits in a fridge for an extended period can develop slightly off tastes (or maybe it’s just my paranoid imagination).