De & Co Water Tank Cleaning
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Guide · 6 min read

How often should you clean a water tank?

A practical answer based on 35+ years cleaning rainwater tanks across Franklin and South Auckland. Short version: once a year for most properties. Longer version below, including the warning signs that mean don't wait.

The short answer

For most rainwater-fed homes in New Zealand, every 12 months is the sweet spot. That schedule keeps sediment from building up to the point where it starts moving water quality and clogging filters, and it's the rhythm we recommend to virtually every household we work with around Waiuku and the wider Franklin district.

There are properties that can stretch to 18–24 months and properties that need a clean every 6–9 months. The difference comes down to four things: what's coming off your roof, what the tank's made of, how the water tastes, and how the rest of the system is set up. Let's go through each.

What's coming off your roof

Every rainwater tank is downstream of a roof. That roof — and especially the trees over it — is the single biggest driver of how fast sediment builds up.

  • Open, clean roof, no overhanging trees: 12-month cycle, no problem.
  • Some trees nearby, a few leaves into the gutter: 12 months, plus a good gutter clean each autumn.
  • Heavy bush cover, leaves in the gutter constantly, lichen on the iron: 6–9 months. Sediment builds up faster than you'd think.
  • Native trees with sticky drop (kahikatea, kauri): Same as above. The resinous drop is harder to keep on top of.
  • Birds nesting in the gutters: Whatever the schedule, sort the birds first.

Roof condition matters too. Old painted iron flakes paint; new Zincalume can shed a fine grey wash for the first six months. Both end up at the bottom of the tank as sediment.

What the tank is made of

Tank material doesn't change the cleaning frequency much, but it does change what we find when we open the lid:

  • Plastic tanks are the easiest to clean and slowest to develop biofilm on the walls. 12-month cycle works fine.
  • Concrete tanks hold more biofilm on the walls and slowly leach calcium over time. We see them get away with longer cycles, but they're also the tanks most likely to need a repair check during the clean.
  • Stainless steel stays cleanest of all on the inside. Sediment still settles at the bottom though.
  • Fibreglass — older tanks especially can develop a slimy biofilm faster. Annual cleans are worth it.

How the water tastes

Your tongue is a surprisingly reliable sediment monitor. If the water tastes different from how it normally tastes — earthier, metallic, slightly sour, or noticeably "off" — the tank is overdue, full stop.

Same for smell. A faint earthy note is normal; a strong sulphur smell (rotten egg) usually means anaerobic bacteria have got established in the sludge layer. That's a clean-and-sanitise job, not a "wait six months" job.

And if the water is visibly cloudy out of the tap, or you're constantly having to clean filters or unblock taps, the tank is well overdue.

How the rest of the system is set up

A tank with a properly maintained first-flush diverter, leaf guards on the inlet, and a screen on the overflow will go longer between cleans than a tank with none of that. We can fit those on most setups during a routine clean — they don't cost much and they slow sediment build-up significantly.

UV systems and filtration also affect things. If your UV is working and filter cartridges are getting changed on schedule, you've got more headroom. If the UV bulb hasn't been replaced in three years (they have a service life), you don't.

So when's the right time to book?

Most of our customers book a clean in late spring or early summer — tank's been refilling all winter, weather's good for the job, and you're heading into the busiest water-use months of the year with clean water.

Late autumn (after the leaves have stopped falling) is the other sensible time. The next 6–8 months of clean rain has a fresh start.

Bottom line: if you can't remember when it was last cleaned, it's overdue. Call Hank on 021 781 618 and we'll have a chat about timing and rough cost.

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