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If you fix one number in deep water culture, fix this one. Reservoir temperature sits upstream of root rot, dissolved oxygen, algae, bolting, and most of the mysterious mid-summer collapses that make people give up on hydroponics.
It is also the variable home growers are least likely to be measuring.
The target
65 to 68F (18 to 20C). Tolerable from about 62 to 72F. Below 60F and above 75F you are actively losing.
Note that this is water temperature, not air temperature, and in a lit indoor system the water is often several degrees warmer than the room. A comfortable 72F room can easily hold a 78F reservoir under a hot fixture.
What goes wrong when it is too warm
Three things happen at once, which is why warm water is disproportionately destructive.
Oxygen supply falls. Water at 85F holds roughly 25% less dissolved oxygen than water at 60F.
Oxygen demand rises. Root respiration and microbial activity both accelerate with temperature. So the supply curve and the demand curve move apart together, and the gap opens faster than the raw saturation figures suggest.
Pathogens accelerate. Pythium, the water mould behind almost all hydroponic root rot, reproduces slowly below 68F and rapidly above 72F. Warm, low-oxygen water is precisely the environment it needs, and stressed roots are precisely the tissue it can colonise.
What you see: wilting despite a full reservoir, slowed growth, tan or slimy roots, a sour reservoir smell, and in lettuce and spinach, bolting and bitterness.
What goes wrong when it is too cold
Less dramatic, and worth knowing so you do not overcorrect.
Below roughly 60F, root metabolism slows and nutrient uptake drops. Phosphorus is affected first and most, which shows up as purpling stems, dark leaves and stalled growth that looks exactly like a phosphorus deficiency. Adding phosphorus will not fix it.
Cold water is not dangerous the way warm water is. Nothing is colonising the roots. Growth simply stops, and resumes when it warms.
Measure it properly
A cheap aquarium thermometer left permanently in the reservoir is one of the highest-value purchases in a hydroponic setup.
Two things people get wrong. First, they measure at the top of the reservoir, where a lid-mounted probe reads the warmest water. Measure mid-depth. Second, and more importantly, they spot-check in the evening. Reservoir temperature peaks near the end of the light cycle, and a bucket that reads a comfortable 68F when you check it at 9pm may have spent six hours at 76F that afternoon.
If you can log it continuously rather than spot-checking, do. The daily swing tells you more than any single reading, and it is usually wider than people expect.
Where the heat is coming from
- The grow light, radiating directly onto the lid and bucket walls. Usually the largest single contributor.
- Submersible pumps. An in-reservoir circulation pump dumps essentially all of its electrical draw into the water as heat. In a small reservoir this is not trivial. Use an external inline pump where you can.
- Room temperature, obviously, and the reservoir will always trend above it under lights.
- Small volume. Less water heats faster and swings harder. Volume is thermal inertia.
- Direct sun through a window, on any part of the reservoir, at any time of day.
- Warm floors and shelves. Buckets on a heated floor or a shelf near ceiling height run warmer.
Fixes, cheapest first
- Move it. Off the shelf, onto a concrete floor, out of the sun. Free, and frequently enough on its own.
- Wrap it. Reflective bubble insulation or panda film, reflective side out, taped around the bucket. Blocks radiant heat from the fixture and blocks light for algae at the same time.
- Bucket in a bucket. Nest the reservoir inside a second bucket. The trapped air gap is genuine insulation for the price of a spare bucket.
- Increase volume. Move from a five gallon bucket to a twenty gallon tote, or link buckets into one shared reservoir. More water swings less.
- Frozen bottles. Two sealed two-litre bottles swapped morning and evening will hold a five gallon bucket a few degrees down. Labour-intensive and effective. Use sealed bottles, never loose ice, which dilutes the solution and can add chlorine.
- A fan across the water surface. Evaporative cooling can pull a few degrees for the cost of a USB fan. It also increases water loss, so expect more frequent top-ups.
- Run lights at night. Shifting the photoperiod to the coolest hours costs nothing and can be worth several degrees in a room without air conditioning.
- A water chiller. The only permanent fix. A 1/10 HP aquarium chiller rated for around 40 gallons handles a modest multi-bucket setup comfortably, and units at this size usually ship with the circulation pump and hose you need to plumb it in. Expensive and worth it if you are running several buckets through a hot summer, and unnecessary if you are running two jars of lettuce in a basement.
The order to try things
Move it, wrap it, then add volume. Those three are free or nearly free and they solve most home setups. Frozen bottles get you through a heatwave. Buy a chiller only when you have done the first three and are still above 72F at peak. At that point it stops being an indulgence and becomes the cheapest thing you own, measured against the crops it saves.
And measure before and after each change. Temperature is one of the few variables in hydroponics where you get an unambiguous number telling you whether the thing you just did worked.