We use ORP to watch the bleach working, not as a chlorine number. Bleach raises it; as the chlorine reacts and gets used up, the reading comes back down. That's why we add regular unscented bleach (6–8.25% sodium hypochlorite) at 0.2 mL per US gallon when we mix and again every 4 days until we change the reservoir, dosed on the volume left. If ORP drops, check pH before adding anything; if it keeps disappearing fast, look for contamination.
What ORP tells you
ORP is oxidation-reduction potential, measured in millivolts. It shows the oxidizing condition of the solution. It isn't a chlorine ppm reading or a dissolved-oxygen reading, and a good number doesn't prove every surface in the system is clean.
Why it falls
Chlorine reacts with organic material and other substances in the water, and those reactions use it up. Root material, returning irrigation water and contaminated equipment keep adding demand after the reservoir is mixed. Warmth, light and storage time also shorten how long it lasts. That's why a fresh reservoir and a dirty one respond differently to the same dose, and why one dose doesn't last the life of a reservoir.
Bleach at mixing and every 4 days
Dose each addition on the solution actually in the tank. A full 100-gallon batch takes 20 mL; with 50 gallons left at the next 4-day dose, that addition is 10 mL. Full routine and the dose math: our bleach routine.
This is our sterile soluble program. Skip it with Granular or beneficial microbes. Another sterilizer has its own strength and directions; the same milliliters don't deliver the same treatment from every bottle.
Read ORP alongside pH
A higher pH lowers chlorine's ORP response even while free chlorine is still there. A healthy, nitrate-fed recirculating crop pushes pH up as it feeds, and CO₂ leaving the water adds to it, especially with splashing, surface agitation or pump heat. Correct the pH drift before you decide a lower reading needs more bleach.
We use wavemakers to circulate and mix pH additions below the surface, keep unnecessary agitation and heat down, and keep the oxygen supply to DWC roots.
RunClean controls scale and works as pH down. It isn't the oxidizer and doesn't raise ORP. Never combine concentrated RunClean with bleach; it can release chlorine gas.
When the reading drops fast
Look for contamination in the reservoir and equipment. More bleach won't remove attached residue or recover a compromised pump. In our experience, black aquarium pumps and air stones that biofilm has gotten into can't be sterilized and need replacing. Replace them, clean the rest of the system and remix. A good reading in the open water doesn't mean the inside of a pump is clean. See white film, sediment or slime.
If the reading itself jumps around, clean and check the ORP probe per its instructions. That's a check on the meter, not the first explanation for visible contamination.
Why a dose can't predict the millivolts
Bottle strength and dose tell you how much chlorine you added. They don't tell you the ORP increase in your nutrient solution; pH, temperature and whatever the chlorine reacts with all change it. To compare products, treat portions of the same finished feed at the same pH and temperature and compare the change from each starting reading. That's a one-off comparison, not another daily log.
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