On any system with a storage tank, whether rainwater, a well with a holding tank or a gravity fed supply, the filter's position matters as much as its specification. Filtration before the tank and filtration after it do different jobs, and the arrangement most people fit protects neither the tank nor the household as well as it could.
Filtration before the tank
This protects the tank itself and everything stored in it. Keeping sediment out means the layer on the floor stays thin, so the tank needs cleaning far less often and does not release a slug of accumulated silt whenever demand is heavy. On rainwater it also keeps organic material out, which is what feeds growth and makes stored water go bad. A coarse screen and a first flush arrangement do most of this work cheaply, and they are the highest value components in the whole system.
Filtration after the tank
This protects the household and the plumbing, and it is what actually determines water quality at the taps. It has to deal with whatever the tank itself contributed: fine sediment stirred from the floor, biofilm, and anything that grew during storage. Draw from a floating intake taking water from below the surface film and above the sediment layer rather than from the tank floor, then filter coarse to fine, then carbon, then disinfect if the water is for drinking.
Why disinfection has to come after
Stored water can be recontaminated at any point in the tank, so treating water on its way in and then storing it achieves very little. Ultraviolet placed after the tank and close to distribution is the usual arrangement, and it depends entirely on the filtration ahead of it, since suspended particles shadow organisms from the lamp and dissolved iron and organics absorb the wavelength. Chlorination is the alternative where clarity cannot be guaranteed, and it does leave a residual in the tank.
The tank itself
Keep it sealed, light proof and insect screened, because algae need only daylight and mosquitoes need only a gap. Fit a calmed inlet so incoming water does not stir the sediment layer. Leave an accessible hatch, since these need cleaning periodically no matter how good the pre filtration is. And where the water is aggressive, as rainwater is, choose plumbing materials accordingly. Our whole house filter guide covers sizing the stages after the tank.
Questions people ask about water filter for water tank
Should the filter go before or after the tank?
Both, ideally. Before the tank protects the tank and what is stored in it; after the tank determines quality at the taps.
Where should water be drawn from the tank?
A floating intake below the surface film and above the sediment layer, rather than from the tank floor.
Can I disinfect before storage?
Not effectively. Stored water can be recontaminated in the tank, so disinfection belongs after it and close to distribution.
How do I stop things growing in the tank?
Keep it sealed, light proof and insect screened, fit a calmed inlet, and keep organic material out with pre filtration.