Ever seen ORP only rise with time?

SlowAndStupid · Apr 16, 2025

I am currently dosing ozone nightly for the purpose of water clarity, currently running for 1 hour overnight. I have an ORP probe monitoring levels for the purpose of not letting it get too high due to concerns for livestock in the system.

What I have noticed since starting my dosing is that the ORP goes up as expected but then never trends back down after finishing the hour of ozone. I have checked the probe with a calibration solution and it reads appropriately and if I put the probe into a rodi water solution the orp trends down as expected as well. Water changes also show a quick drop in Orp before heading back up so the probe appears to be functioning as it should.

I have now slowly trended up from an orp of around 300 to currently sitting at 430 today. I don't want to keep doing my hour of ozone and kill my fish but I do like the improved water clarity. Has anyone seen ORP not trend down after dosing like this? What I see from forum searches is the orp should trend back down once you turn off the ozone.

Randy Holmes-Farley · Apr 16, 2025

The ro/di test has no utility, since it will not read on an ORP probe well, nor do we know what it should be, but yes, ORP continuing to rise with daily 1 h additions is not unexpected.

I might either reduce the amount or the time of dosing.

SlowAndStupid · Apr 16, 2025

Randy Holmes-Farley said:
The ro/di test has no utility, since it will not read on an ORP probe well, nor do we know what it should be, but yes, ORP continuing to rise with daily 1 h additions is not unexpected.

I might either reduce the amount or the time of dosing.

I guess my expectation was for the ORP to rise over the period of an hour and trend back down for the next 23 hours and repeat itself. Other graphs seem to suggest as much. Instead its just a continued stairstep upwards. Chemically what causes the ORP to remain elevated despite holding the dose?

I guess what I will do next is stop the ozone for a period and monitor the trend.

Randy Holmes-Farley · Apr 16, 2025

SlowAndStupid said:
I guess my expectation was for the ORP to rise over the period of an hour and trend back down for the next 23 hours and repeat itself. Other graphs seem to suggest as much. Instead its just a continued stairstep upwards. Chemically what causes the ORP to remain elevated despite holding the dose?

I guess what I will do next is stop the ozone for a period and monitor the trend.

Ozone tends to oxidize things like trace elements that directly impact ORP. Unless somethign is tending to reduce them again, they will just stay oxidized and ORP will stay up. I've never intermittently delivered ozone, so don't know what to expect, but extrapolating from other tanks may not give a full story in your system.

In any case, clear water is the goal, not any particular ORP.

ORP also rises if algae grows on ORP problems.