Algae in Water Tanks: Prevention and Fixes

A green tinge inside your backup tank is one of those problems that starts small and gets quietly worse. Algae needs only two things to thrive: light and nutrients. In a Johannesburg garden or garage, both are often available in abundance — and once a colony establishes itself, it can affect water taste, clog filters, and create conditions that encourage bacterial growth. The good news is that algae in a water tank is almost entirely preventable, and a current bloom can be cleared with straightforward effort.

Why algae grows in the first place

Algae are photosynthetic organisms — they convert light into energy, using dissolved nutrients in the water as building blocks. In a backup tank, those nutrients come from dust, organic debris, and even the water itself if it has been standing for a while. Johannesburg’s high UV intensity and warm summers make conditions particularly favourable.

The single biggest cause of algae in residential tanks is inadequate light-proofing. Translucent polyethylene tanks — the common white or pale-blue variety — allow enough diffuse light through the walls to sustain algae, especially when positioned in direct sun. Tanks with poorly sealed lids, uncovered vents, or cracked fittings are equally vulnerable. If you can see daylight from inside the tank, algae can photosynthesise.

Why it actually matters

A greenish tank is not simply an aesthetic issue. Algae growth has a few practical consequences worth understanding:

• Taste and odour. Algae produce compounds that give water an earthy or musty smell and an unpleasant flavour. This becomes noticeable even after the water has been filtered.
• Bacterial risk. Algae mats create low-oxygen microenvironments where anaerobic bacteria — including some harmful strains — can shelter. South Africa’s drinking-water quality benchmark, SANS 241 (2024 edition), sets strict limits on microbial contamination; algae growth makes it harder to stay within those limits without additional treatment.
• Filter and pump damage. Algae fragments and the biofilm they produce can clog sediment filters faster and coat pump impellers, reducing performance and shortening equipment life.
• Disinfection interference. If you dose stored water with chlorine or other sanitisers, algae and organic material consume that disinfectant rapidly, leaving less available to do its actual job.

Preventing algae: light control is the priority

The most effective prevention measure costs very little: eliminate light entry entirely.

Choose or convert to an opaque tank. Dark-coloured polyethylene tanks — black or dark green — block significantly more light than white or translucent ones. If you already have a pale tank, black exterior paint formulated for plastic can help, though it is not as reliable as a tank manufactured to be opaque from the start.

Seal every opening. Lids should fit tightly with no gaps. Inlet and outlet pipes need proper fittings — no daylight should enter around them. Overflow pipes and vents should be fitted with insect- and light-excluding screens.

Position matters. Where you have a choice, position the tank away from prolonged direct sunlight. Shade from a structure is preferable to relying entirely on the tank’s surface properties.

Keep it clean. Organic material entering the tank feeds algae even in reduced light. Fit an inlet filter screen and inspect it regularly. Read our full guide on cleaning a water tank for a step-by-step routine, and for advice on how long stored water stays usable, keeping stored water fresh covers the key points.

For a broader look at how tank storage fits into a complete backup water strategy, the backup water systems guide covers the full picture.

What to do if algae is already present

If the tank has already gone green, cleaning it out is unavoidable — treatment alone without physical removal is not reliable.

1. Isolate the tank. Turn off the inlet and do not use the water for drinking or cooking until the tank has been treated and retested or flushed thoroughly.
2. Drain completely. Open the outlet or use a pump to empty all water. Dispose of it on garden areas away from stormwater drains.
3. Scrub the interior. Using a stiff brush (not a wire brush on plastic), scrub all interior surfaces to remove algae films and any biofilm. Pay attention to corners, the floor, and around fittings.
4. Sanitise. Mix a dilute chlorine solution — household bleach at a concentration appropriate for tank sanitisation (your installer or a water treatment supplier can confirm the correct dosage for your tank volume) — and apply it to all interior surfaces. Allow contact time as directed, then drain and rinse thoroughly with clean water. Repeat the rinse until no chlorine odour remains.
5. Inspect and fix the light path. Before refilling, identify and fix whatever allowed light in. Without fixing the root cause, the algae will return.

If your tank keeps going green despite cleaning, the issue may be with the installation itself. Compare 3 free quotes — vetted installers, no obligation.

Ongoing maintenance to stay ahead of the problem

Algae prevention is not a once-off task. A sensible maintenance schedule for a backup tank in Johannesburg conditions includes:

• A visual inspection every three to six months, checking for light entry, cracks, and unusual colour or odour in the water.
• A full drain and clean at least annually, following the process described in our cleaning a water tank guide.
• Inlet screen checks after heavy rain or dusty periods, which are common on the Highveld.
• Water quality checks if the tank feeds any point-of-use intended for drinking. Basic test kits are available from hardware and water-treatment suppliers and can flag microbial or chemical issues before they become a health concern.

Preventive action	Effort	Approximate frequency
Check lid and fittings for light gaps	Low	Every 3 months
Clean inlet screen	Low	After heavy rain or dust events
Full interior scrub and rinse	Moderate	Annually, or after any algae bloom
Inspect tank exterior for cracks	Low	Every 6 months
Water quality test (if used for drinking)	Low–moderate	Annually or after contamination risk

A note on chemical treatments

Some homeowners ask whether algaecides or ongoing chlorine dosing can replace physical cleaning. The honest answer is no — not reliably. Chemical treatments can suppress algae in the short term, but they do not remove the biofilm and organic material that accumulate on tank walls. Overdosing creates its own risks to water quality and to the tank material itself. Ongoing chlorine dosing, if done carefully and at the correct level, can form part of a maintenance plan, but it works best alongside good physical hygiene, not instead of it.

Thinking about upgrading to a properly specified, light-proof tank system? Get free quotes from local installers — no obligation, no sales pressure.

The bottom line

Algae in a water tank is a preventable nuisance that becomes a genuine problem if left unaddressed. The fix is unglamorous but effective: block the light, keep the tank clean, and inspect it regularly. In Johannesburg, where backup water systems are increasingly standard rather than optional, a well-maintained tank is worth the modest effort it takes to keep it that way.