Cleaning a Water Tank: How Often and How

A backup tank is only as useful as the water inside it. Most Johannesburg homeowners invest in storage to ride out supply interruptions, but the tank itself introduces a maintenance obligation that is easy to ignore until water comes out smelling wrong. This guide covers how often to clean, what the process actually involves, and what to watch for so your stored water stays usable rather than becoming a health risk.

Why cleaning matters more than most people expect

Static water is not the same as flowing tap water. Once water sits in a closed tank, several things happen: fine sediment settles on the floor and walls, biofilm — a thin layer of bacteria — begins to form on surfaces that stay wet, and in tanks exposed to any light, algae can establish a foothold. None of this is immediately visible, which is why tanks that look fine from the outside can contain water that is well outside what SANS 241 (2024 edition), South Africa’s drinking-water quality standard, considers acceptable for human consumption.

The 27-hour citywide storage buffer that Johannesburg’s reservoir system operates on means municipal supply is vulnerable to relatively short disruptions. Many homeowners supplement with tanks sized for several days to weeks of household use. The longer water sits, the more critical regular cleaning becomes.

How often should you clean a water tank?

As a general guide:

• Every six months for tanks used for drinking or cooking water, or any tank that refills and empties frequently.
• Every twelve months at a minimum for tanks used primarily for toilet flushing, garden irrigation, or other non-potable purposes.
• Immediately if you notice any of the following: visible sediment or discolouration, an unusual smell, slime on the walls when you inspect the interior, or after a period when the tank sat completely stagnant for more than a few months.

These are rough guides — the right interval depends on your tank material, its position (underground tanks accumulate different contaminants than above-ground ones), and whether animals or insects can access the inlet or vent.

Tank use	Suggested cleaning interval
Potable (drinking/cooking)	Every 6 months
General household (non-potable)	Every 12 months
After extended stagnation	Immediately before use
After flood or contamination event	Immediately

What you will need before you start

Gather the following before draining anything:

• Rubber gloves and eye protection
• A stiff-bristled brush that can reach the tank walls (not metal-bristled on plastic tanks)
• A wet/dry vacuum or submersible pump to remove the final rinse water
• Food-grade disinfectant or unscented household bleach (sodium hypochlorite)
• Clean water for the final rinse — ideally municipal supply rather than borehole water, which may introduce its own contaminants before you have finished cleaning
• A torch for inspecting the interior

Avoid using soap, detergent, or any scented product inside a tank. These are difficult to rinse out completely and can contaminate the next fill.

The cleaning process, step by step

1. Drain the tank completely. Use the drain valve if fitted. A submersible pump speeds this up considerably on larger tanks. Do not attempt to clean with water still sitting inside — you will just spread the sediment around.

2. Remove loose sediment. Once empty, use your brush or a damp cloth to loosen any sediment or deposits on the floor and lower walls. Scoop or vacuum this out before introducing any cleaning solution.

3. Prepare your disinfectant solution. A typical dilution for household bleach (at around 3–5% sodium hypochlorite) is roughly 1 litre of bleach per 1,000 litres of tank capacity, mixed with enough water to allow you to coat all internal surfaces. Always confirm the correct concentration for your specific product — packaging or a water-quality professional can advise.

4. Scrub all internal surfaces. Work systematically from the top down — walls, corners, floor, and around any inlet or outlet fittings. Pay attention to the waterline, where biofilm tends to be thickest.

5. Let the solution dwell. Leave the disinfectant solution in contact with all surfaces for at least 30 minutes before draining.

6. Rinse thoroughly — more than once. Drain the disinfectant solution, then rinse the tank with clean water at least twice. After the final rinse, smell the interior. A faint chlorine smell will dissipate; a strong chemical odour means you need another rinse.

7. Inspect before refilling. Use your torch to check for any remaining residue, damage to the lining, or cracked fittings. Replace inlet screens or covers if they are damaged — these stop insects and debris getting in between cleans.

For a broader look at how tank cleaning fits into your overall water independence setup, the backup water systems pillar covers tank sizing, pump selection, and system design in one place.

Keeping water fresh between cleans

Cleaning is a reset, not a permanent fix. Between cleans, a few habits extend the quality of what you store:

• Keep the tank covered at all times. Exposed water grows algae quickly in Johannesburg’s sunshine.
• Maintain regular turnover — tanks that sit full for months without being drawn down stagnate faster than tanks that cycle regularly.
• Consider a small in-line UV or chlorine dosing system if you are storing water for drinking purposes for extended periods.
• Check your stored water for smell and clarity every few weeks, not just at cleaning time.

For more detail on keeping stored water in good condition day to day, the post on keeping stored water fresh covers practical steps you can take without specialised equipment.

If you are installing a new tank or replacing an old one, getting the right setup from the start makes cleaning easier and water quality better. Compare 3 free quotes — vetted installers, no obligation.

When to call a professional rather than DIY

Most residential tanks under around 5,000 litres can be cleaned by a capable homeowner following the steps above. Consider professional cleaning if:

• The tank is large, underground, or confined-space entry is required — this carries real safety risk without the right equipment.
• You find significant structural damage, cracks, or signs that rodents have accessed the tank.
• Water quality remains poor after cleaning, which may indicate a problem with incoming water rather than the tank itself.
• You are preparing the tank for potable use after it has only been used for non-potable purposes.

Professional tank cleaning services typically include a post-clean water-quality test, which gives you documented confirmation that the water meets SANS 241 before you drink it. Pricing varies — get two or three quotes and ask specifically whether water testing is included.

Costs to keep in mind

DIY cleaning costs are mostly your time plus the price of bleach or a food-grade disinfectant — a modest outlay. Professional cleaning services will vary depending on tank size and access; as a rough guide, expect to pay more for larger tanks and confined-space work, and confirm costs via quotes before committing.

If your tank is part of a broader backup water system — including a pump and pressure vessel — the pump should be serviced at the same interval. Backup tank-and-pump systems typically cost R20,000–R40,000 installed, with whole-house configurations running around R55,000, so protecting that investment with regular maintenance is straightforward economics.

A clean tank is only half the picture — the pump and filtration setup matter just as much for usable water. Get free installer quotes — vetted professionals, no obligation.

The short version

Clean your drinking-water tank every six months, your non-potable tank annually, and any tank immediately after a contamination event or long stagnation period. Drain fully, scrub with a dilute bleach solution, rinse at least twice, inspect before refilling. Keep the tank covered and the water cycling between cleans. If you are in any doubt about water quality, test it against SANS 241 before using it for drinking or cooking — the standard exists precisely because water that looks clean can still be unsafe.