Rising PH & ORP

Reflective · Jun 1, 2015

So my tank is pretty new, but since it cycled the PH has been pretty steady between 8.10 & 8.20 and the ORP was steady around 390 to 410 for the past month. I did a 20% water change a few days ago and noticed the ORP steadily rose up to 470 over the next 8-12 hours and has hovered around there since. Today the PH rose up to 8.3 which I realize is still within an acceptable range, but it's just higher than I've seen it go before.

Just wondering what would cause the ORP to rise and if that is connected to the PH rising? I could find reasons for the ORP lowering, but not rising. Is it just the unstableness of a new tank or something else?

Randy Holmes-Farley · Jun 2, 2015

The ORP rise is not connected to the pH rise. A pH rise is usually accompanied by an ORP drop.

ORP often drops on a water change simply because the trace elements in the mix may be in a more reduced state (example, ferrous iron rather than ferric iron).

ORP often rises when an ORP probe gets dirty, so you might carefully clean it. An ORP of 470 mV seems on the high side for a tank not using oxidizers such as ozone, but ORP never actually gets "too high" without oxidizers.

pH is determined solely by the alkalinity and the CO2 level. If the CO2 levels in the system (often driven partly by the CO2 level in your home air) is steady, then perhaps the pH rose due to an alk increase from the water change, or perhaps the pH probe needs to be recalibrated.

Reflective · Jun 2, 2015

Thanks! I will try cleaning/recalibrating

Randy Holmes-Farley · Jun 2, 2015

You're welcome.

Let us know what happens.

Reflective · Jun 24, 2015

So I finally got my calibration fluid! Haven't used it yet though.

While I was waiting, I did clean off the ORP probe but the numbers didn't change so I figured it needed calibration. However, about two weeks ago I started dosing Triton Elementz and think I have just locked in the dosing amount for the 8 dKH about 2 days ago. After a couple days of dosing the ORP started slowly dropping and the past 4-5 days it has stayed consistently between 370 and 390. The pH never went above 8.35 but for the past 5 days it's stayed between 8.23 and 8.35 in a 24 hour period. The highs are when my T5's are on from 12pm-5pm.

So I think the dosing has kept everything much more consistent. There isn't much in my tank so I didn't think anything was being used in between water changes, but I think I was wrong about that!