Between burst pipes, 54-hour planned outages and municipalities dosing extra chlorine after coliform scares, Gauteng households have spent 2025 and 2026 leaning hard on stored water. That means more people than ever are lifting a tank lid and finding something green, murky or smelling like a farm dam. Almost all of it is preventable — and most of it is fixable without draining thousands of litres.
Why has my tank water gone green?
Green water is algae, and algae need exactly two things: light and food. A translucent tank, a lid that doesn’t seal, an uncovered inspection port or a clear fitting lets in the light. Dust, pollen and bird droppings washed off your roof supply the food.
That’s why green water is a diagnosis, not bad luck. Somewhere, light is getting into your water. Dark-coloured tanks with tight lids can hold water for months without a hint of green — find the gap, close it, and the algae lose their power supply.
Why does my tank water smell stale or rotten?
Smell usually starts at the bottom, not the top. Every rainfall washes fine debris into the tank, where it settles into a sediment layer. Bacteria feed on that layer, and in still water the bottom few centimetres run out of oxygen — that’s where the swampy, rotten-egg character comes from.
The first rain after a long dry spell is the worst offender: months of dust, droppings and leaf matter arrive in one concentrated slug. A first-flush diverter dumps that slug on the ground before it reaches your tank — a simple fitting at the downpipe that needs no power and no attention once it’s in.
How do I stop tank water going green or stale?
Four layers, cheapest first:
- Seal out light. Dark tank, tight lid, cover any clear ports. No light, no algae — it really is that binary.
- Screen every opening. A lid strainer with fine mosquito mesh keeps out leaves, insects and light in one go. At R179 it’s the best rand-per-litre protection on the tank.
- Divert the first flush. Stop the dirtiest water of every rain event at the downpipe, before it becomes bacteria food.
- Clean once a year. Sediment still builds slowly even on a well-screened tank; remove it before it turns.
Keep the gunk (and the light) out
How often should I test my tank water?
SANS 241 is South Africa’s national drinking-water standard, and it’s the benchmark to read your results against. You don’t need a lab every month — a home test kit handles routine screening. Work off this calendar:
| When | What to run | Why |
|---|---|---|
| After the first rains | Bacteria test + full strip panel | The season’s first run-off is the dirtiest water your roof will ever send you |
| Mid-summer | Strip panel + visual check | Peak heat and light mean peak algae risk |
| Before winter | Strip panel + TDS | Confirm the water is sound before the dry months when you rely on it most |
| After any repair or municipal outage | Bacteria test | Work on your system — or on the mains — can introduce contamination |
One timing note: home bacteria tests take 24–48 hours at room temperature. Test before you need the answer, not the morning you plan to drink the water.
Honest note: test strips and home bacteria kits are screening tools, not SANAS-accredited lab results. They’ll flag when something is off; they won’t certify your water as SANS 241 compliant. If the water is for drinking — especially for babies, elderly family or anyone immunocompromised — confirm any worrying result through an accredited lab.
How do I clean a tank without wasting a full tank of water?
- Time it. Clean at the end of the dry season when the level is naturally low — you want to throw away sludge, not water.
- Rescue what’s usable. Pump or siphon the clear upper water into drums, or straight onto the garden.
- Isolate the outlet. Close the outlet valve so stirred-up grit doesn’t wash into your pump or pipes.
- Scrub, then flush. Work the walls and floor with a long-handled brush, push the sludge towards the outlet and flush it out — the standard 40mm bottom outlet on a JoJo shifts sludge quickly.
- Refill and test. Run a bacteria test on the first refill before anyone drinks it.
When is green water recoverable — and when should I drain?
Recoverable: a faint green tinge, no strong smell, no visible mats or floating clumps. Close off the light source, treat the water, and keep using it for the garden and washing while you retest.
Drain it: established algae mats on the walls, thick black sludge, a sewage or dead-animal smell, or a bacteria test that stays positive after treatment. Half-measures on a badly fouled tank waste more water in repeat treatments than one proper clean-and-refill does.
Can I treat tank water with household bleach?
Yes — carefully, and only with plain, unscented bleach. Nothing “thick”, lemon-fresh or with added cleaners. Small, measured amounts of chlorine are the standard way to knock back bacteria and algae in stored water: follow the water-treatment dosing on the product’s own label, mix it through, close the lid and give it time to work. More is not better — overdosing just leaves you with water nothing can use.
Remember that bleach treats the symptom, not the cause. If light and debris keep getting in, you’ll be dosing again next month. And treated water isn’t drinking water until a bacteria test comes back clear — factor in that 24–48 hour wait.
Test it, then clean it properly
FAQ
Is green tank water dangerous?
Mostly it’s a taste and smell problem, but green water tells you light and nutrients are getting in — the same conditions bacteria thrive on, and some algae types can produce toxins. Treat it as non-drinking water until it’s been cleaned up and tested.
My tank smells of chlorine after a municipal top-up — is something wrong?
Usually the opposite. During the 2026 water disruptions, municipalities added extra chlorination after coliform detections, so a chlorine whiff after refilling from the mains generally means the disinfectant is doing its job, and it fades within a day or two.
How long does a home bacteria test take?
24–48 hours at room temperature. Strips for pH, chlorine and hardness read in minutes, but anything involving coliforms or E. coli needs incubation time — plan around it.
Will a lid strainer really stop mosquitoes?
Yes. Fine mosquito mesh blocks the insects, keeps leaves out and cuts the light that feeds algae — three problems, one R179 fix.
WaterMart launches in August 2026 with everything above — strainers, first-flush kits, test kits and cleaning brushes — couriered anywhere in South Africa for a flat R85, free over R950, prices including VAT. Join the waitlist now and have your tank sorted before the next outage notice drops.