You lift the reservoir lid and smell rotten eggs. This is not "the water went stale" and not a ventilation question. The smell of hydrogen sulfide is a specific chemical signal that somewhere in the system there is a zone with no oxygen at all, where bacteria have switched to breathing sulfate instead. The crop may still look fine, and that is exactly what makes the smell useful: it arrives before the symptoms on the plant do.
Quick glossary: Hydrogen sulfide (H₂S) - a gas smelling of rotten eggs, produced by reduction of sulfate under oxygen-free conditions; toxic to roots at concentrations a person can barely detect. Sulfate-reducing bacteria - anaerobic bacteria that use sulfate in place of oxygen and give off hydrogen sulfide. ORP - oxidation-reduction potential of the solution in mV, showing which way the system's chemistry is tilted.
Where the Sulfur Comes From If You Never Added Any
The first question people ask about this smell is where the sulfur came from at all. The answer is in the recipe. Magnesium in hydroponics is almost always supplied as magnesium sulfate, potassium often as potassium sulfate, and the sulfate anion comes along with them. In a working solution it runs around 200 mg/L, and that is a normal figure rather than a mixing error.
While the system stays aerobic, sulfate simply sits in solution and serves as the plant's sulfur source. The moment oxygen runs out somewhere, sulfate becomes an alternative electron acceptor for bacteria, and hydrogen sulfide comes out the other end. So a rotten egg smell does not mean you poured in the wrong thing. It means an oxygen-free zone has appeared in the system.
Smell as a Diagnosis
The nose distinguishes more than people expect, and this is the fastest way to narrow the search before you reach for an instrument.
| What you smell | What it means | How urgent |
|---|---|---|
| Rotten eggs | hydrogen sulfide, sulfate reduction, a deep anaerobic zone | stop and strip it down today |
| Sour, fermented | organic matter fermenting: dead root, substrate debris | find and remove the organics this week |
| Swampy, putrid | bacterial root rot | inspect roots now, see Root Rot |
| Ammonia | nitrogen cycle failure, typical in bioponics | fix nitrification, do not disinfect |
| Earthy, mushroomy, faint | ordinary microbial background of a living system | normal, do nothing |
That last row matters as much as the first. A reservoir with living microflora smells of soil, and trying to drive it to the smell of chlorinated water costs more than it returns.
Then confirm with an instrument. The reference points from the dedicated ORP article: below 200 mV the environment is already reducing and Root Rot is either present or days away; negative ORP is precisely the state in which hydrogen sulfide and methane appear. Measure DO alongside it: below 2 mg/L conditions are anaerobic by definition.
Where the Oxygen-Free Zone Forms
Aeration of the reservoir as a whole can be perfectly adequate while one particular spot has held no oxygen for months - dead legs and abandoned pipe runs, the bottom under the pump, dead root mass, an oversaturated substrate, or the space under a biofilm. Look where water stands, not where it flows.
Dead legs and abandoned pipe runs. A section cut off after a rebuild, a bypass line with a closed valve, an elbow below the drain level. Water never leaves any of them.
The reservoir floor under the pump. Sediment collects in a layer the flow never reaches. The visual sign is distinctive: black sludge rather than brown. The black color is iron sulfide, the product of hydrogen sulfide reacting with iron from the recipe.
Dead root mass. The most common source in systems that run several cycles without a full strip-down. Root from a previous batch rots in the channel and creates an oxygen-free zone around itself.
The body of the substrate. An overwatered slab or cube where water has displaced the air. Outside it is wet and unremarkable; inside it is already anaerobic.
Underneath biofilm. This is the main reason the problem comes back. A film on a pipe wall has an aerobic upper layer and an anaerobic lower one, and while the film is intact the zone beneath it lives its own life regardless of how much oxygen is in the flow.
What to Do, in Order
Sequence matters more than choice of product here, because every oxidizer is consumed by organic matter, and the more mass you leave behind, the less of it reaches the target.
Drain first and look. A solution carrying hydrogen sulfide is not rescued, it is replaced. Draining also makes the reservoir floor visible, and with it the black sediment if there is any.
Remove organics mechanically. Sediment, dead root, slime from walls and pipes. This is the work that produces the result; everything after it only consolidates.
Deal with the dead legs. An abandoned pipe run is not disinfected, it is removed. While it stays in the system the problem will return every time.
Sanitize clean surfaces. The protocol with doses and contact times is covered in the article on system sanitation.
Restore oxygen and hold the temperature. Aeration lifts DO and ORP together. Warm water holds less oxygen while simultaneously speeding up microbial activity, so solution temperature is not a secondary parameter here.
A separate word on personal safety. In a reservoir of a few dozen liters the hydrogen sulfide concentration is low and presents no hazard. A large sealed tank or a sunken sump is a different situation, and nobody goes in there without ventilating first. At higher concentrations hydrogen sulfide deadens the sense of smell, so a fading smell inside an enclosed space is not good news.
Three Mistakes That Cost the Most
Dosing peroxide and leaving everything else as it was. The smell goes away for a day, because peroxide oxidizes the hydrogen sulfide that already formed. The zone producing it stays where it is and starts again. Peroxide here removes a symptom and never touches the source.
Turning up aeration and considering the matter closed. A compressor raises oxygen in the body of the solution and reaches neither under a biofilm, nor into a dead leg, nor inside a waterlogged slab. That is precisely why the problem recurs in systems with "powerful aeration".
Waiting for the smell before acting. By the time hydrogen sulfide is detectable, the anaerobic zone has been working for a long while. Regular inspection of dead legs and the reservoir floor, rather than a reaction to the smell, is the difference between prevention and stripping the system down mid-cycle.
How to Know the Reservoir Is Sound
There is no rotten egg smell, and what smell there is reads earthy and faint. ORP holds above 400 mV and DO above 5 mg/L, both measured rather than estimated. There is no black sediment on the reservoir floor and no leftover root from the previous batch in the channels. Dead pipe runs are physically absent rather than "shut off with a valve". After a full sanitation the system returns to working figures in days rather than weeks, and the smell does not come back before the end of the cycle.
If you want to go deeper: ORP / Redox: What Solution "Cleanliness" Means and When It Matters puts a numerical scale under what you have just diagnosed by smell, and Biofilms in Hydroponics: Why the System "Grows Over" and How to Stop It explains why an anaerobic pocket survives disinfection and comes back in the same spot.