Additional approved RO storage can help with a burst of drinking-water use after it fills, but it does not increase the membrane's production rate. Confirm whether the actual system permits a larger or second tank, its usable capacity and the installation requirements. Diagnose a tank that does not fill or deliver properly before buying extra storage.
Extra storage is a model-specific option
Whirlpool's two-tank guidance says multiple storage tanks can be used with its RO arrangement and directs the installer to the owner's manual. That is not blanket approval to tee any tank onto every RO system.
Ask the manufacturer to identify an approved tank, connection arrangement and applicable commissioning steps for your exact model. The installer should confirm the pressure limits, shutoff operation, appropriate support and access. Use a tank approved for this drinking-water RO system rather than choosing by connection size alone.
A gallon label is not necessarily usable drinking-water volume
The APEC TANK-3 specification provides a holding-capacity table at stated inlet pressures and a specified precharge. Those conditions belong with the capacity comparison. The name alone does not promise the same draw volume at every home pressure.
The Pentek RO-3000/RO-3500 manual similarly describes pressure-dependent storage capacity and how drawing water causes the system to replenish it. Use the actual tank/system data rather than assigning these specifications to another model.
Changing the air charge to chase extra capacity is not a sizing method. Tank assessment requires the manufacturer's empty-water-side procedure. Leave an unfamiliar or malfunctioning tank to qualified service.
More storage does not make the membrane produce faster
A tank stores water the membrane has already made. Additional storage needs time to accumulate that supply; it does not increase the installed membrane's production rating. For that specification, compare membrane GPD with complete-system daily production.
Describe the household's bursts of use and the time available between them. A storage change may address a short peak, while sustained demand can require a separate system-capacity assessment. Obtain the manufacturer's recommendation rather than buying the largest tank that fits the cabinet.
Rule out an existing fault before adding equipment
If the tank never accumulates water, begin with tank-not-filling checks. If it contains water but delivery is weak, use full-tank flow troubleshooting. Extra storage will not automatically repair a closed valve, restricted delivery path or failed bladder.
Keep the approved parts and revised layout with the maintenance record. The modified arrangement still needs its correct leak, shutoff, flushing and performance checks before returning to service.