Tank Water Smells Bad: Causes and Solutions

Stored water that smells unpleasant is one of the most common complaints from Johannesburg homeowners who have installed backup tanks — and it is almost always fixable. The smell is your tank telling you something has changed inside it. Before you reach for a treatment product, it pays to understand exactly what that smell means, because different causes need different solutions.

Why Tank Water Develops a Smell

Water stored in a tank is no longer moving. Municipal water relies partly on continuous flow and residual chlorine to stay fresh; once it sits in a sealed vessel, that protection fades quickly. Johannesburg’s climate — warm summers, intense UV, frequent heat fluctuations — accelerates the process. A few things happen when water stagnates:

• Residual disinfectant depletes. The small amount of chlorine or chloramine left in municipal supply typically dissipates within 24–72 hours in a warm tank, leaving nothing to suppress microbial growth.
• Bacteria multiply. With no disinfectant and a nutrient-rich environment (dust, organic matter, biofilm on tank walls), bacteria establish colonies. The byproducts of their metabolism produce odour compounds.
• Algae and organic matter accumulate. Any tank exposed to light — or one with a poorly fitted lid — can develop algae, which contributes earthy or musty smells.

The specific smell gives you a useful diagnostic clue.

Reading the Smell: What Each Odour Usually Means

Smell	Likely cause	Starting point
Rotten egg / sulphur	Sulphate-reducing bacteria producing hydrogen sulphide	Inspect for biofilm, check if tank has been stagnant for days
Musty or earthy	Algae or mould growth	Check for light penetration, inspect lid seals
Chemical or chlorine-like	Residual disinfectant reacting with organics, or a cleaning product residue	Flush tank thoroughly, review cleaning process
Fishy or oily	Organic decomposition, sometimes associated with algae die-off	Full clean and inspection required
General staleness	Prolonged stagnation without turnover	Increase usage frequency or circulation

None of these smells on their own confirm that the water is safe or unsafe to drink. The only way to know is to test it.

The Risks of Ignoring the Problem

Smelly water is not automatically dangerous, but it is a warning sign worth taking seriously. The National Institute for Communicable Diseases (NICD) recognises waterborne bacterial contamination as a real public health concern in South African households, particularly where water is stored without adequate treatment or hygiene controls. Bacteria such as E. coli and Legionella can be present in tank water without producing any obvious smell at all — which is precisely why smell alone is not a reliable safety indicator.

South Africa’s drinking-water quality standard, SANS 241 (2024 edition), sets the benchmark for what constitutes safe potable water. Water stored in a private tank is not automatically tested against this standard; that responsibility falls to the homeowner. If you are using tank water for drinking, cooking, or brushing teeth, independent laboratory testing is the only credible way to confirm it is safe.

How to Fix Smelly Tank Water

Step 1 — Empty and clean the tank. Most problems originate in biofilm or sediment on the tank walls and floor. Drain the tank completely, scrub internal surfaces with a food-grade disinfectant solution (typically a diluted chlorine-based cleaner — follow the product manufacturer’s dilution rate), rinse thoroughly, and allow to drain fully before refilling. Do this at least once a year; twice is better in Johannesburg’s summer heat.

Step 2 — Inspect and seal the tank. Light penetration encourages algae. Check that your lid fits tightly, that vents are screened to prevent insects and debris entering, and that the tank is opaque or shaded. Our detailed guide on algae in water tanks covers this in more depth if green growth is your specific problem.

Step 3 — Improve water turnover. Stagnation is the enemy. If your tank fills but rarely empties — for example, you filled it during a supply disruption that then resolved — the water can sit for weeks. Try to use and replace stored water on a regular cycle rather than treating it as purely emergency reserve.

Step 4 — Consider a dosing or filtration system. For tanks used for drinking water, a low-level chlorine dosing system, UV purifier, or multi-stage filtration setup (particulate, carbon, then disinfection) can maintain water quality between cleans. Talk to an installer about what suits your tank size and usage pattern. Our broader resource on backup water systems explains how these components fit together.

Step 5 — Test before you drink. Submit a sample to a SABS-accredited laboratory before drawing on tank water for consumption after any corrective action. Testing costs are modest and the result gives you a factual baseline rather than a guess.

If your tank setup needs a filtration or dosing upgrade, getting a few independent prices makes a real difference. Compare 3 free quotes — vetted installers, no obligation.

Keeping the Smell from Coming Back

Prevention is simpler than remediation. A few habits make a significant difference:

• Schedule tank cleans. Put a reminder in your calendar — once before summer, once before winter is a sensible rhythm for Johannesburg.
• Keep first-flush diverters in good working order if your tank is fed by rainwater harvesting. First-flush devices discard the initial, most contaminated roof runoff before it reaches the tank.
• Store water correctly. UV-stable, opaque polyethylene or fibreglass tanks fare better than translucent ones in direct sunlight. Tank position and colour genuinely matter.
• Understand safe storage duration. Our article on keeping stored water fresh goes into the practical timelines and rotation approach in detail — worth reading if you want a systematic maintenance routine.
• Check inlets and outlets. Cracked inlet pipes, corroded fittings, or inadequate screening can introduce contamination continuously, making cleaning efforts short-lived.

When to Call a Professional

If the smell returns quickly after a thorough clean, if your water tests positive for bacterial contamination, or if you are unsure what type of tank system you have and how it connects to the rest of your plumbing, get a qualified installer or water-quality technician to assess it in person. Some smells — particularly persistent sulphur odours — can indicate a deeper issue with the tank lining or a slow contamination source that a visual inspection alone will not reveal.

Backup water storage is a sensible investment in Johannesburg’s supply environment, but it introduces responsibilities that municipal supply normally handles for you. Staying on top of tank hygiene is the main one.

Not sure whether your current tank setup is adequate — or thinking about upgrading to include proper filtration? Get free quotes from local specialists — no commitment required.

A Quick Note on Costs

If your smelly tank situation leads you to conclude that your current setup needs replacing or upgrading, it helps to have a rough sense of scale. Backup tank-and-pump systems typically run R20,000–R40,000, with whole-house configurations around R55,000 as a rough guide — though your actual quote will depend on tank capacity, pump requirements, filtration components, and installation complexity. Always confirm current pricing with at least two or three local suppliers before committing.