Storage tanks are usually thought about at the outlet, where the household's water quality is decided, and the inlet is where the cheapest improvement is available. Every particle stopped before the tank is one that will not settle into a sludge, will not feed growth in the dark, and will not have to be shovelled out when the tank is eventually cleaned.
What inlet filtration prevents
Sediment accumulating on the tank floor, which is where the outlet then draws from every time the pump starts. Organic material settling, which feeds bacterial growth in water that has no disinfectant residual. And the gradual loss of usable volume as the sludge layer deepens. A tank that has never had inlet filtration is a tank that will eventually need emptying and washing out, which is a considerably larger job than fitting a screen was.
What to fit, by source
On a rainwater system, a first flush diverter that discards the initial run off from each event plus a leaf and grit screen, which together remove most of what the roof contributes. On a hauled or well fed cistern, a spin down separator or a coarse cartridge, sized for the fill rate rather than the household draw. On a mains topped tank, a simple screen is usually enough. In all cases the inlet filter is coarse: fine filtration belongs on the outlet where flow is controlled.
Keep light and vermin out too
Filtration is only part of it. Light entering the tank grows algae, so tanks should be opaque and covered, and a translucent tank in a bright plant room will go green regardless of what is filtered. Every opening including the vent needs screening against insects and vermin, since a mosquito or a rodent entering contaminates the water directly, past any filter. Those two measures cost very little and prevent problems that filtration cannot address.
The outlet still decides what you drink
Inlet filtration keeps the tank clean; it does not make the water potable. Stored water has no disinfectant residual and sits in the dark, so the outlet train still needs sediment, carbon and disinfection where the water is drunk, with the disinfection stage placed after the tank so anything the tank contributed is dealt with. The EPA notes private supplies are not federally regulated, so testing is the owner's. Our whole house filter guide covers the outlet train.
Questions people ask about filter water tank
Why filter into a tank as well as out of it?
Everything kept out never settles as sludge, never feeds growth, and never has to be cleaned out later.
What should an inlet filter be?
Coarse: a first flush diverter and screen on rainwater, a spin down or coarse cartridge on a fed cistern. Fine filtration belongs on the outlet.
Does filtering the inlet make the water safe?
No. Stored water has no residual, so the outlet train still needs disinfection, placed after the tank.
What else should the tank have?
An opaque cover to keep light out, and screening on every opening including the vent, against insects and vermin.