This is for AutoPot growers adjusting pH every day and watching it walk away again. Start by making sure you're measuring the right water, because a tray reading tells you about the substrate, not the tank. Once the reading comes from the reservoir, drift traces back to something in or around the tank, and each cause below has one fix.
The short answer
- Test and adjust in the reservoir only. Aim for 5.8 with soil, 5.5 to 5.6 with coco.
- Pull any air stone out of the tank.
- Close the tap before refilling, and give the tank 30 to 60 minutes before opening it again.
- Test any additive in a bucket for a few days before it goes in the tank.
- Keep the lid on and light off the solution.
Where to measure, and what to aim for
AutoPot USA says to measure and adjust the reservoir, not the tray. Tray solution has already passed through substrate and roots, so its reading is skewed by the pot, and adjusting the tank to correct it chases the wrong number.
| Growing medium | Reservoir pH |
|---|---|
| Soil or soil blends | 5.8 |
| Coco or coco blends | 5.5 to 5.6 |
An AutoPot tank only sends solution out; nothing drains back from the trays. So plant uptake can't move reservoir pH. When the tank reading drifts, the cause is the water, what you added to it, or the conditions around the tank.
The refill-and-settle routine
Refill when about a third of the solution is left. Adjusting pH at refill, and only then, works when the tank is allowed to settle first.
- Tap off. Close the tap at the tank so fresh water doesn't push stirred-up sediment into the lines.
- Refill and mix. Add water and nutrients, then set pH to your target.
- Wait 30 to 60 minutes. AutoPot's settle time lets sediment drop back out of suspension.
- Check, then open. Take one more reservoir reading and turn the tap back on.
A full, settled tank is also the right moment to open the flush taps at the ends of each line; the routine is in keeping your AutoPot clean between runs.
What's pushing the pH
An air stone in the reservoirpH creeps up between refills.
- Cause. AutoPot's guide says not to put an air stone in the reservoir because it can raise pH.
- Fix. Take it out. If you want the solution moving, AutoPot lists a water pump as optional, and its own cleaner instructions warn against constant circulation and aeration because they swing pH. A pump on a timer, not running nonstop, keeps within both.
Hard, alkaline tap waterYou bring pH down at refill and it climbs back within the tank.
- Cause. Water from limestone areas carries bicarbonate and carbonate, which raise pH and buffer against acid. Part of each pH Down dose goes into neutralizing that buffer, so the reading rebounds.
- Side effect. AutoPot notes hard water reacts with nutrients and forms sediment, and says to flush pipework more than once a week in hard water areas.
- Fix. Get your source water tested for pH, EC, and alkalinity. University of Missouri Extension lists 40 to 160 ppm alkalinity, measured as calcium carbonate, as the optimum range for hydroponic water. Above that, blending in lower-alkalinity water such as RO reduces the buffer you're fighting.
Organics or additives sitting in the tankpH moves after you add a product, or cloudiness and sediment appear.
- Cause. AutoPot USA says organic and organic-leaning nutrients can drift in pH sitting in a standing reservoir.
- Test. Mix a bucket at normal strength and let it sit a few days, watching pH and sediment. A product that drifts in the bucket will drift in the tank.
- Fix. Run a water pump in the reservoir for 15 minutes every 2 hours. When the tank is almost empty, clean it, discard the leftover solution, refill with pH-stable plain water, run that for 12 to 24 hours, then feed again. If a product still won't hold, it belongs in the tray; see feeding an AutoPot.
Light reaching the solutionGreen film on the tank walls, or pH that climbs faster in a translucent tank.
- Cause. Algae photosynthesis in nutrient solution raises pH, a pattern documented in peer-reviewed greenhouse hydroponics research.
- Fix. Oklahoma State University Extension recommends covering hydroponic tanks with lids or black plastic so light can't reach the solution. Keep the tank lid on, and follow AutoPot's advice to place the reservoir outside the growing area.
A warm reservoirTank solution above 70°F.
- Cause. AutoPot's target is 64 to 70°F, because dissolved oxygen drops quickly above 70°F. Oklahoma State Extension also lists warm, calm water among the conditions algae need, which ties heat back to the card above.
- Fix. Move the tank off heat sources and out of the grow area, and shade it from light.
A tray reading that's high while the tank is fineNot reservoir drift at all.
- Cause. Clay pebbles and Growstones in the pot have to be pH stable. Some pebble brands aren't and raise tray pH, and Growstones are alkaline until stabilized.
- Fix. Leave the reservoir alone. Use pH-stable pebbles or stabilized Growstones in the drainage layer at the next potting up.
Monitoring without chasing
AutoPot recommends water monitoring devices for temperature, pH, and conductivity. Log the reservoir reading at each refill, change one thing at a time from the cards above, and let the log show which fix held. There's no standard daily drift rate to compare against; it depends on your water, nutrients, additives, and temperature.
Parts that fix this
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pH and TDS meters
For the one reading that matters: the reservoir at refill.
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pH adjusters
pH Up and Down for the refill step; never mix the two together.
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RO units and filtration
Lowers source-water alkalinity when hard tap water keeps pushing pH back up.
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AutoPot Reflective Cover for Hard Tank
Fits the 12.4 gal tank and reflects light and heat away from the solution.
Questions
After. Add all base nutrients and additives first, since each one shifts pH, then adjust the finished mix. Never combine pH Up and pH Down directly.
No. AirDomes sit in the pot and push air into the root zone; AutoPot's air stone warning is about the reservoir. The two are separate parts of the system, and pulling the air stone doesn't mean switching off your domes.
AutoPot's clean-and-discard step applies to organics, additives, and boosters. On a mineral-only program, its instruction is to refill at about a third left and never let the tank run empty.
Then the problem is mechanical, not chemical. Height, level, silicones, and air in the tray are covered symptom by symptom in why your tray isn't cycling.