To prevent algae in a water tank, keep light out. Algae cannot grow in the dark, so an opaque tank with a tight lid stays free of it. If your tank walls are already green, the tank needs to be drained, scrubbed and disinfected with chlorine, and then the light has to be blocked or the algae will be back within weeks.
Green tanks are a common call-out on Karachi rooftops in the hot months. This guide explains why it happens, whether it is dangerous, and the fixes that last.
The Causes of Algae Growth
Algae are simple plant-like organisms. Their spores are in the air, in dust and in almost all surface water, so they reach every tank. Whether they grow depends on three things.
Light
This is the deciding factor. Algae make their food by photosynthesis. In a tank they get light through:
- thin or light-coloured plastic walls that glow in the sun
- unlined fibreglass
- a missing, cracked or badly fitting lid
- an open inspection hatch or uncovered vent
A concrete underground tank with its cover on almost never grows algae. A translucent rooftop tank almost always does.
Warmth
Algae grow fastest in warm water. A rooftop tank in a Karachi summer is ideal. The same tank in January grows far less.
Nutrients
Algae feed on nitrates and phosphates, which come from sewage, soil, bird droppings and rotting organic matter. Sludge on the tank floor is a steady supply. Water that has mixed with sewage in the supply line, which researchers have documented in Karachi (Dialogue Earth), arrives with fertiliser already in it.
Stagnation
Water that sits for days in an oversized or little-used tank gives algae time to establish. See choosing the right size water tank.
How to Recognise Algae
- Green, brown or black film on the walls, thickest on the sunny side and near the waterline
- Water with a green tint
- Slippery walls and floor
- An earthy, musty or fishy smell and taste
- Green slime in filter cartridges, tap aerators and toilet cisterns
White crust is scale, not algae, and is dealt with differently. See hard water stains and deposits.
The Impact of Algae in Water Tanks
Is algae in a water tank dangerous?
Most algae that grow in household tanks are not poisonous, and a green tank is not an emergency in the way sewage contamination is. It is still a problem that should be fixed, for five reasons.
- It feeds bacteria. Living and dead algae are food. A tank with algae carries far more bacteria than a clean one, and the algal film gives them a surface to shelter in.
- It uses up chlorine. Any disinfectant in the water is consumed by the algae instead of protecting you.
- Some types produce toxins. Blue-green algae, properly called cyanobacteria, can release toxins that affect the liver and nerves. Serious blooms are mainly a problem of lakes and reservoirs, but the organisms can grow in any warm, lit, nutrient-rich water. Boiling does not destroy these toxins.
- It ruins taste and smell. The earthy taste comes from compounds that people can detect at extremely low levels.
- It clogs things. Strands and slime block filters, float valves, aerators and shower heads.
Dense blue-green scum, a paint-like green surface or a strong musty smell deserves prompt action. Stop drinking the water until the tank has been cleaned. For the wider picture of what grows in tanks, see bacteria in Karachi’s water tanks.
Removing Algae from a Tank
- Drain the tank.
- Scrub or pressure-wash every surface, including the underside of the lid and the corners. Algae holds on tightly, and anything left will regrow.
- Remove all the dirty water and loosened growth. Do not let it run down into the house pipes.
- Disinfect with chlorine at the proper dose and contact time. See how to disinfect a water tank after cleaning.
- Rinse and refill.
- Flush the pipes by running each tap, and wash or replace filter cartridges.
- Fix the light problem the same day.
Our crews do this with a high-pressure washer and a chlorine-based disinfectant as part of a standard cleaning. The steps are on the service procedure page.
What not to do
- Do not pour in algaecide or copper sulphate. Pond and pool products are not for drinking water.
- Do not just add bleach to a green tank. It bleaches the top layer, leaves dead algae to rot, and the growth returns.
- Do not use acid. It does nothing useful against algae and is dangerous with chlorine.
Preventive Measures
1. Block the light
- Choose an opaque tank. Multi-layer plastic tanks have a black middle layer for exactly this purpose. Hold a torch to the wall of any tank you are thinking of buying. No light should show inside. See choosing the right tank material.
- Keep the lid on and tight. Replace cracked lids.
- Deal with a translucent tank. Paint the outside with a dark undercoat and a light reflective top coat, wrap it, box it in, or shade it. Paint the outside only.
- Cover vents with mesh and a cowl so that air passes but light does not shine straight in.
2. Keep the tank cool
Shade from a canopy or the stair tower lowers the water temperature and slows all growth. See choosing the right location for your tank.
3. Remove the food
Clean the sludge out every 6 months, keep birds off and out of the tank, and keep both tanks clean, because the underground tank feeds the overhead one. See how often to clean your water tank and bird contamination in rooftop tanks.
4. Keep the water moving
Size the overhead tank for about a day’s use so the water turns over.
5. Maintain a chlorine residual if you can measure it
A small, measured chlorine residual discourages regrowth. Pakistan’s drinking water standard is 0.2 to 0.5 mg/L at the consumer end (S.R.O. 1063(I)/2010). Use test strips. Do not guess.
6. Consider an anti-algae coating
For tanks that cannot be kept dark, such as some concrete roof tanks with access hatches, an internal anti-algae coating made for drinking water tanks is an option. We apply one on request. It is listed on our cleaning chemicals page.
Water Quality Testing
You can see algae, so you do not need a laboratory to find it. Testing is useful for what comes with it.
- Bacteria: a lab test for total coliforms and E. coli tells you whether the water is safe. The national standard is none detectable in 100 ml.
- Chlorine: test strips or a pool kit show whether any disinfectant is left.
- Turbidity and colour: a clear glass against a white background is a fair home check.
Test after cleaning if anyone in the house has been ill, or if the algae was heavy and blue-green. See filtration and cleaning in Karachi homes for how filters cope with algae.
Expert Tips and Recommendations
- Clean before summer. A tank that is scrubbed and disinfected in March or April has nothing for algae to restart from when the heat arrives. See the best time to clean your water tank.
- Look at the sunny side first. Growth starts where the afternoon sun hits the wall.
- Check after storms. Most lids are lost in monsoon winds, and a week without a lid is enough.
- Do not ignore the toilet cistern. Green slime there means the tank above has it too.
- Replace, do not rescue, a cheap tank. A thin single-layer tank will regrow algae every season. A layered tank ends the problem.
The monthly routine is in how to maintain your water tank after cleaning.
Frequently Asked Questions
Why is my water tank green inside?
Light is reaching the water, through thin walls or an open lid, and algae is growing on it.
Is green algae in a water tank harmful?
It is rarely toxic in itself, but it feeds bacteria, consumes chlorine and can include toxin-producing blue-green types. Clean the tank and block the light.
Will chlorine kill algae in a tank?
Chlorine kills algae on a surface that has been scrubbed. Poured into a green tank, it only bleaches the outer layer.
Does a black tank prevent algae?
A tank with a black layer in the wall does, because no light gets through. A plain black tank also blocks light but heats the water more, which is why layered tanks with a light outer colour are the better choice.
How often should I clean a tank that keeps growing algae?
Clean it now, fix the light, and then return to the normal 6-month schedule. If you cannot fix the light, expect to clean every 3 to 4 months in summer.
Green Tank? Get It Cleaned and Disinfected
Call or WhatsApp 0313-2796778. We clean rooftop and underground tanks across Karachi, including North Karachi, FB Area and Gulshan-e-Maymar. Rates are on our charges page, and the calculator on the home page gives an instant rate for your house.
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; National Standards for Drinking Water Quality, S.R.O. 1063(I)/2010.
