Brown water in a toilet is one of the more useful symptoms in a house, because the toilet is often where discoloration shows first: the water sits still in the bowl long enough for anything suspended to settle and anything dissolved to oxidize. Working out which of three causes you have takes about two minutes.
Narrow it down first
Run the cold tap in the bathroom into a clear glass. If it is discolored too, the supply is the cause and every fixture is affected. If the tap runs clear and only the toilet is brown, look at the cistern and the pan. Then flush and watch: water that arrives clear and browns over hours is dissolved iron oxidizing in the bowl, while water that arrives brown is carrying particles already. Those two point in different directions.
When the supply is the cause
Dissolved iron is the usual answer on a well, arriving clear and turning orange to brown in contact with air, staining every fixture that holds water. On a mains supply, brown water across the house usually follows main works or a hydrant being used, stirring settled deposit in the pipework, and it clears within hours after running the taps. Manganese gives a blacker cast. Persistent iron needs oxidation and filtration rather than anything at the fixture.
When it is the cistern or the pan
A cistern that has never been cleaned accumulates sediment and biofilm, and an old steel float arm, screws or a corroding fill valve rust into the water sitting in it. Lift the lid and look: a rusty deposit inside the cistern with a clear supply is the whole answer. In the pan itself, brown staining at the waterline that returns after cleaning is usually iron or manganese deposit building up, and in an infrequently used toilet it can be bacterial growth in standing water.
Fixing each one
Clean the cistern and replace corroding metal parts with plastic where the fault is there. For iron in the supply, an oxidizing filter is the treatment, and Penn State Extension notes iron and manganese carry secondary aesthetic standards with recommended maximums of 0.3 mg/L iron and 0.05 mg/L manganese, and that oxidized iron fouls softener resin, so iron treatment belongs ahead of any softener. Our softener guide covers the treatment order.
Questions people ask about toilet water brown
How do I tell whether it is the supply?
Run the cold tap into a clear glass. Discolored there means the supply; clear there means the cistern or the pan.
Water arrives clear and browns in the bowl, what is that?
Dissolved iron oxidizing in contact with air. It needs an oxidizing filter on the supply rather than anything at the fixture.
Could the cistern itself be the cause?
Often. Accumulated sediment, biofilm and corroding steel float arms or fill valve parts all discolor water standing in it.
Where does iron treatment go relative to a softener?
Ahead of it. Oxidized iron fouls softener resin, so iron is removed before the water reaches the ion exchange tank.