What does a continuous drop in ORP between water changes mean?

[xxxenmsy]

My SPS tank has been established for about 6 months, and I’ve noticed a very consistent pattern with my ORP readings:

The day after a water change: ORP peaks at around 380–390 mV.

Daily trend: It steadily drops by about 10 mV each day.

The day before the next water change: It hits a low of around 330–340 mV.

This cycle repeats predictably with every water change interval.

What does this steady daily drop indicate? Does it mean I'm overfeeding, or is it just the normal accumulation of dissolved organic carbon (DOC)?

 

[Justfebreezeit]

Id assume normal degradation between water changes

Unless you are running something like ozone its unavoidable id imagine

 

[Dan_P]

My SPS tank has been established for about 6 months, and I’ve noticed a very consistent pattern with my ORP readings:

The day after a water change: ORP peaks at around 380–390 mV.

Daily trend: It steadily drops by about 10 mV each day.

The day before the next water change: It hits a low of around 330–340 mV.

This cycle repeats predictably with every water change interval.

What does this steady daily drop indicate? Does it mean I'm overfeeding, or is it just the normal accumulation of dissolved organic carbon (DOC)?

That seems like a small change and not likely something that can interpreted. You would need a test like Triton’s NDOC to measure the DOC before and after a water change to determine if there is a connection with ORP values.

 

[EnterName]

ORP is weird...
Mine is really low and drops with water changes. I'm using Nyos Pure salt and highly pure RO/DI. Activated charcoal doesn't help either, it's sometimes appearing to lower ORP even further
Nutrient levels are really low, but I could imagine that DOC levels are high as I rarely do water changes.

 

[Randy Holmes-Farley Verified Community Expert]

Depending on how the trace metals in the salt mix are formulated, water changes can raise or lower orp. Most often, people see it drop since some low ORP metals such as ferrous iron are more soluble than the higher ORP version (ferric iron) and hence are used for solubility.

 

[xxxenmsy]

Depending on how the trace metals in the salt mix are formulated, water changes can raise or lower orp. Most often, people see it drop since some low ORP metals such as ferrous iron are more soluble than the higher ORP version (ferric iron) and hence are used for solubility.

Exactly. Right after a water change, my ORP drops briefly and then rebounds on the second day—which becomes the highest peak of the week.
From that point on, the daily peak drops by about 10 each day until my next water change, where it jumps back up and the cycle repeats. I've tried reducing my feeding, but it doesn't seem to make much difference.
I'm not really sure what this pattern indicates. Does it mean the system's biological processing capacity isn't keeping up, leading to a accumulation of organics (DOC)? I don't feed heavily at all.

 

[X-37B]

New salt water is always in the lower 300's for me when I make it. It will rebound slowly when added to the system, however I run ozone to keep it at 400 so its stable in my system

 

[Randy Holmes-Farley Verified Community Expert]

Exactly. Right after a water change, my ORP drops briefly and then rebounds on the second day—which becomes the highest peak of the week.
From that point on, the daily peak drops by about 10 each day until my next water change, where it jumps back up and the cycle repeats. I've tried reducing my feeding, but it doesn't seem to make much difference.
I'm not really sure what this pattern indicates. Does it mean the system's biological processing capacity isn't keeping up, leading to a accumulation of organics (DOC)? I don't feed heavily at all.

I don’t think interpretations of ORP are particularly straightforward, but I would caution against the myth that lower ORP means dirtier water.

 

[JonasRoman]

My SPS tank has been established for about 6 months, and I’ve noticed a very consistent pattern with my ORP readings:

The day after a water change: ORP peaks at around 380–390 mV.

Daily trend: It steadily drops by about 10 mV each day.

The day before the next water change: It hits a low of around 330–340 mV.

This cycle repeats predictably with every water change interval.

What does this steady daily drop indicate? Does it mean I'm overfeeding, or is it just the normal accumulation of dissolved organic carbon (DOC)?

New mixed marine water can have low orp depending on its salt mix content. For instance Brightwell, that smells some sulfat when newly mixed, has very low orp.
AMI its waste of time measure orp in a tank water. Says nothing.

Only area where I think orp is of interest is control anaerobic processes like denitrification filters by measure orp inside the filter.