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The short answer is every one to two weeks for most recirculating systems. The useful answer depends on your system, how much water each plant has to itself, and what stage the crop is at, and it starts with a distinction a lot of guides skip.
Topping up is not changing
These are two different jobs and conflating them is the root of most reservoir problems.
Topping up replaces water lost to transpiration and evaporation. Plants take up water faster than they take up nutrient, so what is left behind gets more concentrated. Top up with plain pH-adjusted water, not nutrient solution. Doing this every few days is normal.
Changing means dumping everything and mixing fresh. This resets the nutrient ratios, which topping up cannot do. Over a couple of weeks plants strip nitrogen and potassium far faster than calcium and sulphate, so an old reservoir can sit at a perfectly normal EC while being badly out of balance. Your meter will not tell you this. Only the calendar will.
By system type
| System | Full change | Notes |
|---|---|---|
| DWC (single bucket) | 7 – 10 days | Small volume per plant, so it drifts fast |
| RDWC (recirculating) | 10 – 14 days | Larger shared volume buffers changes |
| NFT | 7 – 10 days | Thin film, high oxygen exchange, fast drift |
| Ebb and flow | 10 – 14 days | Media buffers somewhat |
| Drip, recirculating | 7 – 14 days | Check emitters at every change |
| Countertop units | 14 – 21 days | Follow the maker, but do not stretch it |
| Kratky | Never, by design | See below |
Shorten these intervals when the reservoir is warm, when plants are large relative to water volume, or during heavy fruiting. Stretch them slightly for seedlings and slow winter growth.
The volume-per-plant rule
More useful than any calendar: the less water a plant has to itself, the faster that water goes wrong.
- Leafy greens: roughly half a gallon to one gallon per plant
- Herbs: around one gallon per plant
- Peppers, cucumbers: two to three gallons per plant
- Tomatoes: three to five gallons per plant
A mature tomato in a five gallon bucket can drink two gallons in a hot week. That is 40% of the reservoir replaced by top-up water in seven days, and the chemistry moves accordingly. The same plant on a twenty gallon shared reservoir barely shifts it.
If you find yourself topping up daily, your reservoir is too small for the crop. Bigger reservoirs are more stable in every dimension: nutrient, pH, and temperature.
Why Kratky is different
The Kratky method is deliberately non-circulating. You fill once, and as the plant drinks, the water level falls and leaves behind an air gap where the roots grow a dedicated oxygen-absorbing section. Topping the water back up destroys that gap and drowns those roots. That is the most common Kratky failure.
So with Kratky you do not change or top up mid-grow. You size the container to last the whole crop, and if it runs dry early you harvest early or refill below the previous line, never back to the top.
Signs you need a change regardless of schedule
- pH will not hold. If you correct it and it has drifted a full point within 24 hours, the solution is exhausted and has lost its buffering.
- EC climbs on plain-water top-ups. Salts are accumulating.
- Any smell. Fresh solution is odourless.
- Cloudiness, slime, or film on the walls or roots.
- Growth has stalled with everything reading in range. This is the classic signature of nutrient ratio drift.
- You just fought root rot. Change immediately, do not wait out the interval.
How to change it properly
- Mix the new solution first and let it come to room temperature. Dumping cold tap water onto warm roots shocks the plant.
- Drain fully. Leaving a few inches of old solution to save effort carries the imbalance straight into the new batch.
- Wipe the walls while it is empty. Thirty seconds now prevents the biofilm that becomes next month is algae problem.
- Set EC first, pH second. Adding nutrient moves pH, so doing it the other way means doing it twice.
- Refill and check the level against the net pots. In established DWC leave a one to two inch air gap below the pot base.
- Recheck pH after an hour of circulation. It usually settles somewhere other than where you left it.
The whole job takes about fifteen minutes per bucket. Skipping it is the most expensive fifteen minutes you will ever save.
Two things make it faster: a VIVOSUN digital pH and TDS/EC meter set so you are not guessing at EC and pH, and a Humboldts Secret pH Up and Down kit for the correction step.