Most water tank problems in Karachi come down to five causes: sludge on the tank floor, a lid or cover that does not seal, a faulty float valve, a crack or loose fitting, or dirty water arriving from the supply. Find which one you have and the fix is usually simple. This guide lists the symptoms you will notice at the tap or on the roof, traces each to its cause, and tells you what to do and who should do it.
Understanding Your Water Tank
A typical Karachi system has six parts, and a fault in any one shows up somewhere else:
- Inlet from the street main or a tanker hose.
- Underground tank, usually concrete, with a cover at floor level.
- Pump with a suction pipe and foot valve in the underground tank.
- Overhead tank on the roof, with a lid, a float valve or level switch, an overflow and a vent.
- Outlet and distribution pipes down to the taps.
- The taps and appliances themselves.
If you are not familiar with the layout, see underground water tanks and overhead water tanks. Materials and designs are compared in types of water storage tanks.
Common Problems with Water Tanks
| Symptom | Most likely cause | Fix |
|---|---|---|
| Yellow, brown or cloudy water | Sludge stirred up from the tank floor, or dirty supply after a gap | Clean the tank. If it returns within days, the supply is the source. |
| Rotten-egg or sewage smell | Rotting sludge, or sewage entering the line or the underground tank | Clean and disinfect. Check for cracks and for sewage mixing in the street line. |
| Musty or earthy smell, green walls | Algae from light entering the tank | Clean, then block the light: opaque tank, tight lid. |
| Sand or grit in buckets | Tanker or boring water sediment, outlet too close to the tank floor | Clean. Raise the outlet or fit a strainer. |
| Worms, larvae or mosquitoes | Open or loose lid, unscreened vent or overflow | Clean and disinfect. Seal the lid and mesh the openings. |
| White crust on walls and fittings | Hard water scale | Descale during cleaning. |
| Low pressure at all taps | Tank too low, outlet or pipes blocked with sediment or scale | Clean the tank and flush the lines. Raise the tank or add a pressure pump. |
| Low pressure at one tap | Blocked aerator or shower head | Unscrew and soak in vinegar. |
| Overhead tank overflows | Float valve stuck open, or automatic switch failed | Clean or replace the float valve or switch. |
| Overhead tank does not fill | Float stuck shut, pump fault, underground tank empty, blocked foot valve | Check each in turn, starting with the water level below. |
| Pump runs but lifts no water | Lost prime, air leak in the suction pipe, foot valve stuck with sludge | Clean the foot valve, re-prime, check suction joints. |
| Water level drops when nobody is using water | Leak in the tank, a fitting or a pipe, or a dripping cistern | Do an overnight level test, then trace it. |
| Damp patch on the roof or ceiling under the tank | Leaking outlet fitting or cracked tank base | Tighten or replace the fitting. Replace a cracked plastic tank. |
| Rust-coloured water | Steel tank or iron pipes corroding, or iron in boring water | Replace rusting parts. Test the bore water. |
| Very hot water from the cold tap | Overhead tank in full sun | Shade or insulate the tank. |
Detecting Water Tank Problems
A ten-minute monthly check
- Underground tank: lift the cover and shine a torch in. Look at the colour of the water, anything floating, and the smell.
- Cover: is it intact and sealed? Is water or dirt collecting around it?
- Overhead tank: open the lid. Look for green walls, sediment on the floor, insects.
- Lid, vent and overflow: lid tight, mesh in place.
- Float valve: lift it by hand. It should shut the flow off cleanly.
- Outside of the tank and the roof around it: damp patches, drips, white salt trails.
- A glass of tap water: hold it to the light, then smell it.
Is it the tank or the supply?
Fill a clean glass from the incoming line or tanker before the water enters your tank, and another from a tap. If the first is clear and the second is not, the tank is the problem. If both are dirty, the supply is, and your tank is now collecting it. In 2017 the Pakistan Council of Research in Water Resources found 107 of 118 Karachi water samples unsafe (Dialogue Earth), so dirty supply is common.
The overnight leak test
Fill the tank, switch off the pump, close the outlet valve or make sure no water is used, and mark the level. Check it in the morning. A drop means the tank or its fittings leak. If the level holds with the outlet closed but drops with it open and no taps in use, the leak is in the house plumbing. The full method is in how to detect and prevent tank leaks.
Signs that need same-day action
- A sewage smell or grey water after supply resumes
- A dead animal in the tank
- Floodwater in the underground tank
- Several people in the house ill with diarrhoea or fever at once
Stop drinking the water, boil or buy water for the kitchen, and have the tank cleaned and disinfected. See signs your family may be drinking contaminated water.
Fixing Common Water Tank Problems
Dirty water, smell, sediment, insects, algae
These are all cleaning problems. The fix is a full cleaning of both tanks: drain, remove sludge, pressure-wash, vacuum, disinfect, rinse. Then deal with the reason it happened:
- Seal lids and mesh openings. See bird contamination in rooftop tanks.
- Block light. See preventing algae growth.
- Keep to a 6-month schedule. See how often to clean your water tank.
Scale
Scrape and pressure-wash, and use a food-safe descaler on plastic or steel. Never use bathroom acid. See dealing with hard water stains and deposits.
Float valve faults
Most float valves fail because grit or scale stops the washer from seating. Remove the valve, clean it, replace the washer, and refit. If the arm is bent or the float is waterlogged, replace the whole valve. They are cheap.
Low pressure
First clean the tank and flush the pipes, because sediment is the commonest cause. If pressure is still poor on the top floor, the tank is too low. Each metre of height adds about 0.1 bar. Raise the tank or fit a pressure pump on the outlet. See choosing the right location for your tank.
Pump problems
A pump that hums without lifting water has usually lost its prime because the foot valve is jammed open by sludge. Cleaning the underground tank often cures it. For motors that trip, overheat or make noise, see pump system maintenance and call an electrician or pump mechanic.
Leaks and cracks
- Fittings: tighten, re-tape or replace the tank connector and its rubber washer.
- Plastic tank body: a cracked plastic tank cannot be reliably repaired. Replace it.
- Concrete tank: cracks must be cut out and sealed with a waterproofing product approved for drinking water, by a mason who knows the work, while the tank is empty and dry.
Rust
Replace galvanised tanks and iron fittings with plastic or stainless steel. Rust does not stop once it starts.
DIY vs. Professional Help
| You can do yourself | Call a plumber or mason | Call a tank cleaning company |
|---|---|---|
| Monthly checks | Float valve and fitting replacement if you are not confident | Any cleaning of an underground tank |
| Replacing a lid, fitting mesh | Pipe leaks, pressure pump installation | Sludge, smell, algae, insects, sewage contamination |
| Cleaning aerators and shower heads | Crack repair and waterproofing | Heavy scale |
| Overnight leak test | Pump and electrical faults | Disinfection after contamination |
Do not enter an underground tank to investigate a problem. It is a confined space, and the air inside can be unsafe without any smell to warn you. See our safety page. The case for professional cleaning is set out in why hire a professional tank cleaning company.
Our own work is cleaning and disinfection. While the tank is empty, our crew will show you any crack, failed fitting or damaged lid they see, so that you can have it repaired before it refills.
Importance of Regular Maintenance
Nearly every problem in the table above starts small and is cheap to deal with early. A tank that is opened, cleaned and looked at every 6 months rarely surprises its owner. The gains are listed in the benefits of regular water tank cleaning. The routine is in how to maintain your water tank after cleaning, and the consequences of skipping it are in the dangers of neglecting tank cleaning.
Legal and Environmental Considerations
A leaking or overflowing tank wastes water in a city that receives roughly half of what it needs (Geo News). Fixing a stuck float valve is the cheapest water conservation measure there is.
When a tank is cleaned, the sludge and chlorinated wash water should go to a proper drain, not onto the street or into a garden. See waste disposal after tank cleaning. If you supply water to tenants, staff or customers, you are responsible for its quality, and the national standard requires that no E. coli be detectable in drinking water (S.R.O. 1063(I)/2010).
Frequently Asked Questions
Why is my tank water dirty even after cleaning?
If the tank was cleaned properly, the dirt is arriving with the water. Line water after a supply gap, tanker water and boring water can all bring fresh silt within days.
Why does my water smell like rotten eggs?
Bacteria in rotting sludge produce hydrogen sulphide gas. Clean and disinfect the tank. If the smell returns quickly, check for sewage entering the supply.
Why does my overhead tank keep overflowing?
The float valve or automatic switch is not shutting off. Clean or replace it.
Can a cracked plastic tank be repaired?
Not reliably. Patches fail under sun and water pressure. Replace the tank.
How do I know if my underground tank is leaking?
Do the overnight level test. A falling level with no use means water is escaping, and where water can get out, groundwater and sewage can get in.
Get the Tank Cleaned and Checked
If your problem is on the cleaning side of the table, call or WhatsApp 0313-2796778. We work in every part of Karachi, including the older neighbourhoods of Saddar, Garden and Liaquatabad. See the service procedure and charges.
About the author: Ahmar Shoaib is the founder of Karachi Tank Cleaning. He holds a BS in Chemical Engineering from Karachi University and has run the company’s cleaning and disinfection work since 2007. This article follows our editorial policy.
Sources: PCRWR water testing for the Sindh High Court judicial commission (2017), reported by Dialogue Earth; PCRWR figures on Karachi’s water demand and supply, reported by Geo News (May 2025); National Standards for Drinking Water Quality, S.R.O. 1063(I)/2010.
