What is actually in your water tank
Nobody opens the tank. Sediment settles, a film grows on the walls, and the first sign is usually a taste. What to look for, and what a real cleaning involves.
Almost every house in Islamabad has two tanks — one underground where the supply arrives, one on the roof that the motor fills — and almost nobody has looked inside either of them.
Both do the same quiet thing. Whatever arrives in the water settles out and stays: fine grit, sand, whatever came through the line the day a main was being worked on. It sinks, it stacks up, and the water above it looks perfectly clear.
The signs, in the order people usually notice them
A taste, or a smell. Most people notice this first, and most people blame the supply. Sometimes it is the supply. Often it is a tank that has not been opened in years.
Grit in the filter. If you have a filter on a tap or a washing machine inlet, pull it out and look. That is your tank, in miniature.
Cloudy water after the motor runs. Filling the roof tank stirs up what is lying at the bottom of it. Water that is fine all day and cloudy for an hour after the motor is a tank telling you something.
Nothing at all. This is the common one. Sediment builds up slowly enough that nobody ever has a day where the water suddenly seems wrong.
Why the underground one matters more
It is the one nobody thinks about, and it is the one with the bigger problem. It is dark, it is cool, it is often not sealed as well as it should be, and anything that gets in cannot get out. A lid that does not close properly is worth more attention than anything else on this page — go and look at yours.
What cleaning it actually means
Not a hose through the inlet. A tank is cleaned by emptying it completely, getting inside or reaching in, scrubbing the walls and the floor by hand, removing what comes loose rather than pushing it round, then disinfecting and rinsing until nothing remains.
It takes a few hours, and it means no water in the house while it happens. Plan the day: fill what you need first.
How often
We are not going to invent a number for you. It depends on your supply, on whether the lids seal, on whether there is construction nearby.
What we will say is this: if you cannot remember the last time, that is the answer. Go and lift the lid on the roof tank and look in with a torch. It is free, it takes two minutes, and you will know more than any article can tell you.
If it looks fine, put the lid back and check again in six months. If it does not, you have found something worth sorting out before it becomes the reason somebody in the house is ill.