Any household on a storage tank, whether harvested rain, a hauled supply, or a well feeding a cistern, has two filtration positions to think about and they do different jobs. Filtering on the way in protects the tank and keeps it clean. Filtering on the way out protects the household. Most installations need both, and the failure modes of a tank explain why.
Filtering on the way in
The inlet filter keeps material out of the tank, and everything it stops is material that will not be sitting on the tank floor for years feeding growth. On a rainwater system this is where a first flush diverter and a coarse leaf and grit screen sit. On a hauled or well fed cistern it is a sediment cartridge or a spin down separator. Inlet filtration is cheap and it is what keeps a tank from needing to be emptied and cleaned every couple of years.
Filtering on the way out
The outlet is where the household's water quality is decided, and the train is the familiar one: sediment first, then carbon for taste and organics, then disinfection where the water is drunk. Draw the outlet a little above the tank floor so settled sediment is not pulled into the line, and fit a foot valve or strainer. This is also where a pump usually sits, and it is worth having a coarse filter ahead of that pump rather than only behind it, so grit does not damage the impeller.
How storage tanks go wrong
Three ways. Light reaching the water grows algae, which is why tanks should be opaque and covered. Insects and vermin entering through a vent or an unscreened inlet contaminate the water directly, so screening every opening is part of the installation. And organic material settling to the floor feeds bacterial growth in water that has no disinfectant residual. Standing water in the dark with a food source is a growth environment, which is why outlet disinfection matters even on a tank fed by treated water.
Maintenance the tank will actually need
Inspect screens and the first flush device seasonally, since a blocked inlet screen diverts water away and an open one lets debris in. Check the tank interior periodically and plan on an eventual clean out, which is easier if inlet filtration has been doing its job. Replace outlet cartridges on schedule, and if disinfection is fitted, test the water after commissioning and periodically afterwards. Our whole house filter guide covers sizing the outlet train for a household's flow.
Questions people ask about filter for water tank
Should the filter go before or after the tank?
Both, ideally. Inlet filtration keeps the tank clean; outlet filtration and disinfection decide what the household actually drinks.
Why does my tank grow algae?
Light is reaching the water. Tanks should be opaque and covered, with every opening screened against insects and vermin as well.
Where should the outlet be drawn from?
A little above the tank floor, so settled sediment is not pulled into the line, with a strainer or foot valve fitted.
Does tank water need disinfection?
For drinking, yes. Stored water has no disinfectant residual and sits in the dark with settled organic material, which supports growth.