When Should You Change Your Water Filter? The Honest Guide

There are two types of people in the world: the ones who change their filter on time, and the ones who never change their filter.

If you are in the second camp, this one is for you. And if you have ever suspected the replacement schedule is just a way to sell more cartridges, fair question. We would ask it too. So here is the science behind the schedules, the test data we used to set them, and what actually happens inside a cartridge that stays on the tap too long. Including why you should replace a filter you have barely used.

The short answer, for every filter we sell. Whichever limit comes first:

EcoPro Compact

Every 2 months or 220 litres

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EcoPro SMR

Every 3 months or 1,200 litres

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Reverse Osmosis Countertop

Every 12 months or 1,200 litres. The unit tells you when.

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ShowerPro Diamond

Up to 6 months per refill

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ShowerPro Rain

Up to 6 months per refill

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And yes, the clock matters even if you barely use your filter. Here is why.

Your filter is a clock, not a fuel tank

It is tempting to think of a filter cartridge like a fuel tank. It only empties when you use it. That is not how activated carbon works.

The moment water first flows through your filter, the carbon inside gets wet. It stays wet. From that point on, three things happen whether you filter ten litres a day or ten litres a month.

1. The carbon keeps working even when you are not using it. Activated carbon filters water by adsorption: contaminants stick to the carbon's surface. That surface has a finite capacity, and it does not pause between uses. The water sitting inside the cartridge keeps interacting with the carbon around the clock. Capacity is used up by time in water, not just litres through it.

2. Your filter removes the thing that keeps water sterile. Chlorine is tap water's built-in disinfectant. Your filter's job is to remove it, and a fresh cartridge removes 98% of it. That is why filtered water tastes so much better. But it also means the water sitting inside a used cartridge between uses has no disinfectant left. It is room temperature, standing still, and in contact with a carbon surface. Past the cartridge's lifespan, those are conditions where microbes can start to grow.

Counterintuitive but true: light use makes this worse, not better. More time between uses means more stagnation.

3. The limescale protection dissolves away on its own. Stage 4 of the EcoPro's filtration is a polyphosphate layer that stops limescale forming in your kettle and on your dishes. It is sacrificial by design. It slowly dissolves into the water over time, regardless of how much you use.

We measured the decay. Here is the curve.

In hard water testing (Barcelona, 324 mg of calcium carbonate per litre):

Notice that is a time axis, not a litres axis. That decay curve is a big part of why the 2-month rule sits where it does.

What actually happens if you do not change it

In roughly the order you would notice:

1. The chlorine taste comes back. As the carbon saturates, chlorine starts getting through. This is usually the first sign, and if you can taste chlorine again, the filter is past done.
2. The invisible stuff gets through silently. Chlorine has a taste. PFAS, pesticides, pharmaceutical traces and lead do not. Their removal declines as the carbon fills up, and you get no warning at all. Still tastes fine is not the same as still protected.
3. Limescale returns. See the table above. By month three you are at 72% and falling.
4. The flow slows down. The carbon block physically captures particles down to 1 to 2 microns. Eventually it starts to clog, and you will notice a weaker or splashier stream.
5. The filter can flip from protecting to harbouring. Left long enough, an old wet cartridge stops being a barrier and becomes a place where bacteria live. Past a certain point, you are better off with no filter at all.

None of this happens on day 61. Filtration degrades gradually, and the schedule includes a sensible safety margin. But the trend only goes one direction, and most of it is invisible.

Why does the SMR get 1,200 litres when the Compact gets 220?

Because the litre rating and the month rating measure two different things.

Litres measure adsorption capacity, and capacity scales with how much carbon you have. The EcoPro SMR uses the full-size cartridge. The Compact packs the same five-stage design into a much smaller body. Roughly five times the filter media means 1,200 litres instead of 220.

Months measure the wet carbon clock. Stagnation, chlorine-free standing water and dissolving polyphosphates do not care how much carbon is in the housing. Which is exactly why the time cap barely moves: 2 months versus 3, while the litre cap moves five-fold.

If replacement schedules were just about capacity, the SMR would be rated ten months. It is not, because time degrades a wet cartridge no matter its size.

One practical note: 220 litres over 2 months works out to about 3.6 litres a day. A bigger household can genuinely hit the Compact's litre cap before the 2 months are up. That is why whichever comes first is not boilerplate.

How do you know a schedule is real? You test the end of life.

When we rated the reverse osmosis cartridge at 1,200 litres, that number came from running a cartridge to 1,200 litres and measuring. Chlorine removal held the whole way. And the membrane's salt rejection was as good at the end of its rated life as at the start: 91.3% at litre zero, 93.7% at litre 1,200.

The chlorine result matters more than it looks. Reverse osmosis membranes are damaged by chlorine. If the carbon stage had worn out early, chlorine would have reached the membrane and rejection would have fallen measurably. It did not. So the carbon was still doing its job at 1,200 litres, and the membrane finished its rated life intact.

The limescale table above is the same discipline applied to the tap filter, run on time instead of volume. Both schedules are backed by measurement, not marketing.

So why can a wet reverse osmosis cartridge last 12 months?

Fair question. If wet carbon gets 2 to 3 months in a tap filter, a 12-month cartridge sounds inconsistent. It is not, for three reasons.

The carbon is not the barrier. The membrane is. In a tap filter, carbon is the last line of defence, and whatever grows on it or slips through it goes straight into your glass. In the reverse osmosis system, the main carbon stage sits upstream of the membrane, and its critical job is stripping chlorine so the membrane is not damaged. Anything that might grow on an ageing pre-carbon then hits a physical wall with pores around 10,000 times smaller than a bacterium, and gets flushed out with the wastewater.

The membrane does not saturate, because it does not adsorb. Carbon fills up. A membrane rejects. Contaminants never accumulate inside it. They are physically turned away and washed down the waste stream.

The carbon after the membrane lives in near-pure water. The polishing stages only ever touch water that has already been stripped of organics, nutrients and microbes. There is almost nothing to feed a biofilm and almost nothing to adsorb. The tap filter's carbon, by contrast, spends its whole life soaking in raw tap water.

In short: the 2 month rule exists where carbon is your only barrier. Reverse osmosis lasts 12 months because its barrier rejects rather than absorbs. And we tested it at month twelve to make sure.

Who tells you when it is time?

The reverse osmosis unit counts for you. It tracks cartridge life and tells you when a replacement is due.

Tap filters cannot do that, and we will not pretend otherwise. The schedule is the indicator. Put it in your calendar, or let us do the remembering: we email you when your replacement is due.

The simplest fix is keeping a year of refills in the cupboard. When the reminder lands, the refill is already there. Twist off, twist on, done.

Staying stocked also protects your warranty. The details are on our warranty page.

The 30 second version

• Change your cartridge on the schedule above. Whichever limit comes first.
• Low usage does not pause the clock. Wet carbon degrades on time, not volume.
• The first sign of an expired filter is chlorine taste returning. The more important signs have no taste at all.
• Keep refills on hand and let our reminders do the remembering.

Questions about your filter's schedule? Email hello@tappwater.com.au, Monday to Friday, 9 to 5 Sydney time.