Storing water that has been filtered is a different proposition from storing water straight from the mains, because filtration usually removes the disinfectant residual along with everything else. Once that residual is gone, the tank is no longer protected chemically, and whether the water stays good depends entirely on how the tank is built, sited and used.
Why treated water is more vulnerable in storage
A chlorinated supply carries a residual that continues to suppress growth in pipes and vessels. Carbon filtration removes that residual, which is the point, and reverse osmosis removes it too. What is stored afterwards has no chemical protection, so any organism entering the tank through a vent, a fitting or a hand has nothing working against it. That is not an argument against filtering; it is an argument for treating a storage tank downstream of a filter as something that needs designing rather than just plumbing.
Materials, light and air
Use tanks made for potable water, since other plastics impart taste and are not intended for drinking water contact. Keep them opaque and out of sunlight, because light is what allows algae to grow and even a translucent tank in a bright plant room will develop it. Screen every opening, including the vent, against insects and vermin. And fit the vent with a filter where the tank breathes in room air, since whatever is in the air enters the tank as the level falls.
Turnover matters more than size
A tank sized far beyond what the household uses holds water for weeks, and water standing at room temperature with no residual is exactly the condition to avoid. Size for a few days of use rather than for reassurance, and arrange the inlet and outlet so the tank actually turns over rather than drawing from the same region each time. Where a large volume is needed for supply reasons, disinfection after the tank, typically ultraviolet, is the standard answer.
Cleaning, and the stage that follows
Plan for periodic cleaning: inspect annually, and clean and sanitize on the manufacturer's method when sediment or biofilm is evident. Draw the outlet a little above the floor of the tank so settled material is not pulled into the line. And where the stored water is drunk, put the disinfection stage after the tank rather than before it, so anything the tank contributes is dealt with. The EPA notes private supplies are not federally regulated, so testing is the owner's. Our hard water shower guide covers a separate point of use question.
Questions people ask about filtered water tank
Why does filtered water not keep as well?
Filtration removes the disinfectant residual, so nothing suppresses growth in the tank or the pipework downstream.
Does the tank need to be opaque?
Yes. Light allows algae to grow, and a translucent tank in a bright room will develop it.
How big should a storage tank be?
Sized for a few days of use rather than for reassurance, so the water turns over instead of standing for weeks.
Should disinfection go before or after the tank?
After, where the stored water is drunk, so anything the tank itself contributes is dealt with before the taps.