Strange drop in ORP every water change

brettonw · Dec 26, 2022

My tank has been having a strange ORP issue, where every time I do a water change it drops to about 250mV and then starts climbing again. It gets to 350-360mV before the end of the week, and then drops again with the next water change. I'm working on tracking down the cause... I thought maybe I'd gotten a contaminant in my water system, so I discarded all my mixed salt water, disassembled and cleaned my entire water production system (including replacing all the pre-filters on my 4-stage RO/DI), and started over. Unfortunately the same thing happened, so I'm down to two variables: the water produced by the RO/DI is not clean enough, or the salt has something in it...

The filtered water appears to be good by any measure I can see. It registers 0 on the TDI meter and 0 to 0.1 on my pinpoint salinity meter. I've had a water test done on this before by the folks who run the Baltimore aquarium and nothing significant was observed, so it's not obviously bad water...

Changing salt is my next step, as it is the cheaper option to start and I suspect that is my problem. I use Tropic Marin Pro Reef salt Mix. I just don't have the ability to mix 200g of saltwater at a time, so I roll the salt in a mixing motion between 10g buckets to make sure it's thoroughly integrated before using. I'm going to try the Red Sea Coral Pro Salt Mix next, after conversations with various folks.

Questions for the room:
- Have you observed problems with the TM salt mix? Does it have some kind of organic compound or filler that would cause this?
- What salt is the current best recommendation? Is Red Sea Coral Pro going to do the job? (I know to transition salts slowly and watch my parms)
- If it turns out not to be the salt, what in the source water could be causing this? If I need to update my filters, I'm going to have to tailor the change to target the precise problem...

Jay Hemdal · Dec 26, 2022

brettonw said:
My tank has been having a strange ORP issue, where every time I do a water change it drops to about 250mV and then starts climbing again. It gets to 350-360mV before the end of the week, and then drops again with the next water change. I'm working on tracking down the cause... I thought maybe I'd gotten a contaminant in my water system, so I discarded all my mixed salt water, disassembled and cleaned my entire water production system (including replacing all the pre-filters on my 4-stage RO/DI), and started over. Unfortunately the same thing happened, so I'm down to two variables: the water produced by the RO/DI is not clean enough, or the salt has something in it...

The filtered water appears to be good by any measure I can see. It registers 0 on the TDI meter and 0 to 0.1 on my pinpoint salinity meter. I've had a water test done on this before by the folks who run the Baltimore aquarium and nothing significant was observed, so it's not obviously bad water...

Changing salt is my next step, as it is the cheaper option to start and I suspect that is my problem. I use Tropic Marin Pro Reef salt Mix. I just don't have the ability to mix 200g of saltwater at a time, so I roll the salt in a mixing motion between 10g buckets to make sure it's thoroughly integrated before using. I'm going to try the Red Sea Coral Pro Salt Mix next, after conversations with various folks.

Questions for the room:
- Have you observed problems with the TM salt mix? Does it have some kind of organic compound or filler that would cause this?
- What salt is the current best recommendation? Is Red Sea Coral Pro going to do the job? (I know to transition salts slowly and watch my parms)
- If it turns out not to be the salt, what in the source water could be causing this? If I need to update my filters, I'm going to have to tailor the change to target the precise problem...

Some salts have a lot of sodium thiosulfate in them. Since you are using RODI, there is no chlorine for it to react with. If it gets into your tank, I’m pretty sure it will lower the ORP for a time.

Jay

brettonw · Dec 27, 2022

Jay Hemdal said:
Some salts have a lot of sodium thiosulfate in them. Since you are using RODI, there is no chlorine for it to react with. If it gets into your tank, I’m pretty sure it will lower the ORP for a time.

Jay

Thank you Jay. try as I might Google isn't very forthcoming about the presence of this compound in the Tropic Marin salt. I'll try asking Tropic Marin...

Jay Hemdal · Dec 27, 2022

brettonw said:
Thank you Jay. try as I might Google isn't very forthcoming about the presence of this compound in the Tropic Marin salt. I'll try asking Tropic Marin...

Sorry, I don’t know if Tropic Marin has it or not. Instant Ocean does, as well as two other brands I tested.
You might also ask if it has EDTA in it. I don’t know if that affects ORP, but it is a chelating agent sometimes added to salts.

Jay

gbru316 · Dec 27, 2022

What detrimental impacts have you linked to temporarily reduced ORP?

brettonw · Jan 9, 2023

@gbru316 My question arose because a measurable change happens in my water chemistry, and I wanted to understand the reason for the change. Unknowns bother me. That said, I've been using TM salt since I set up this tank, so I don't have a "better" baseline for comparison.

@Jay Hemdal TM responded to me they know the salt causes ORP to drop, but don't have a specific cause. They said it is not the result of organics, and speculated that perhaps the iodides cause it. As part of the conversation they recommended aerating the salt water mix before adding it to the aquarium. I rigged my pump hose to vent just above the surface of the water while mixing.

While aerating doesn't seem to have substantially changed the ORP factor, it did take care of a slightly musty smell in the reserve can (I mix about 30 gallons at a time and use it over about 2 weeks on 2 tanks).

I tried using a jug of water from my LFS and observed the same problem, so I no longer suspect my water has something bad in it. The pre-filters go brown after just a few weeks, though. Sigh... Baltimore City man...

I haven't tried a different salt yet due to a minor hurdle when my LFS was out of "small" buckets of salt. I may not bother...

gbru316 · Jan 10, 2023

brettonw said:
@gbru316 My question arose because a measurable change happens in my water chemistry, and I wanted to understand the reason for the change. Unknowns bother me. That said, I've been using TM salt since I set up this tank, so I don't have a "better" baseline for comparison.

@Jay Hemdal TM responded to me they know the salt causes ORP to drop, but don't have a specific cause. They said it is not the result of organics, and speculated that perhaps the iodides cause it. As part of the conversation they recommended aerating the salt water mix before adding it to the aquarium. I rigged my pump hose to vent just above the surface of the water while mixing.

While aerating doesn't seem to have substantially changed the ORP factor, it did take care of a slightly musty smell in the reserve can (I mix about 30 gallons at a time and use it over about 2 weeks on 2 tanks).

I tried using a jug of water from my LFS and observed the same problem, so I no longer suspect my water has something bad in it. The pre-filters go brown after just a few weeks, though. Sigh... Baltimore City man...

I haven't tried a different salt yet due to a minor hurdle when my LFS was out of "small" buckets of salt. I may not bother...

per Randy in an old thread:

ORP often drops due to a water change from, presumably, low ORP ferrous ions and related trace metals in a more reduced but more soluble form in the mix.

IMO, ORP is typically not much of a gauge of health in a reef tank, but sulfur (hydrogen sulfide) smells are undesirable and potentially toxic, and removing the source would both raise ORP and be good for the tank.

Tanxalot · Jul 2, 2023

While I'm not sure this is a problem, I can report a similar effect using HW Wiegandt Reefer salt. Following a recent approx 30% water change, Apex noted ORP drop from 266 to 204. ORP has begun slowly rising and normally ranges daily in this system from 280 to 255.