Effects to Redox and Oxygen Tension

[paulg]

Hello SeaChem;

This is from the "I have Done the Experiments and it May Interest You to Know" department, but there is a question here.

This is a question posed out of scientific curiosity only. I am not troubleshooting. However, my understanding on this subject is not complete, and I would really appreciate your feedback.

I am running a high light/high tech 200 gallon jungle style planted aquarium. I monitor O2 tension (DO) and ORP continuously. Along with lots of other things, there are five large and thriving Echinodorus swordplants. I auto-dose all supplements daily at first light of dawn. In addition to inorganic salts, I dose the following SeaChem substances:

Flourish Comprehensive
Flourish Advance
Flourish Excel
Flourish Trace

All iron in the water column derives from the ferrous gluconate in Flourish Comprehensive. The dosing pump is calibrated such that Fe, usually tested within two hours of dosing, reads 0.1 to 0.3. I try to stay on the high end of this range, currently dosing 8 ml daily. This is more or less in line with your dose-rate recommendation, bumped up a bit for high-growth conditions (carbon fixation proceeds apace).

Excel and Trace are dosed according to your label directions, adjusted for daily distribution but holding to recommended maximums. Trace and Comprehensive doses are separated by 20 minutes. I dose Advance at 50 ml per day (half of label direction).

Inorganic supplements in use, and dosed at the same time as the above, are:

K2CO3
KNO3/Ca(NO3)2
MgSO4/K2SO4

Orthophosphate derives completely from fish food/environmental metabolism and is not dosed as a supplement. This aquarium is heavily stocked.

At the time of dosing - between 07:00 and 07:30 AM - the following two things occur:

1) The ORP plunges approximately 40 mV over about 45 minutes, then gradually recovers throughout the day, plateauing at maximum overnight. The normal running average redox in this tank is currently circa 500 mV.

2) The DO of course descends overnight, but just before lights ON takes a pronounced dip, about 1 ppm. Photosynthesis begins and drives the DO to saturation by midday. The nadir of this dip corresponds exactly with the redox notch. I have programmed aeration to kick in at 6 ppm O2. The air pump is ON-OFF active for around 3 hours. This compensates the O2 deficit and erases the dip in the DO curve, and also appears to accelerate ORP recovery. This period just after dosing is the only time the air pump ever runs.

Clearly, dosing is adding reduced/reducing chemistry. I have experimented with timing the withholding of doses of the various supplements and discovered the following:

Flourish Comprehensive, Excel, and Advance each contribute significantly to this effect. Individually they are slightly variable from day to day, but always evident. Flourish Trace also causes a small but consistently detectable depression in ORP, which I did not expect to see. The inorganic compounds listed above do not seem to exhibit the effect, but if there is a theoretical reason why any of these salts should do so, I will take a second look.

So I am reviewing the ingredients labels and trying to understand what is happening. Again, I wish to be clear about this; there is nothing amiss going on here. DO in this tank never drops below 5 ppm, environmental metabolism is robust, and the BOD is extremely low. I just want to have a more detailed grasp of the chemistry. I am expecting (out of imperfect knowledge to be sure) that in order to identify substances which are reduced, I would typically - but not necessarily exclusively - be searching for organic molecules. I am seeing gluconate, protein hydrolysates (peptides and amino acids), mannitol, ascorbic acid, and polycyclogluteracetyl; and a lot of anions, especially ferrous iron in quantity (which is suggestive). Flourish Trace appears to be all inorganic salts, mostly sulfates, with no complexing agents (chelates), and does not contain any organic compounds.

I think I can generalize a finding that all the named SeaChem Flourish products, taken together, constitute a veritable cocktail of reduced compounds and metals. That adding them to the water column definitely causes the stated reactions in my instrumentation is observed fact.

Which ingredients are actually causing these reactions in my DO and ORP probes?

Paul G

[Tech Support AN]

Hello SeaChem;
So I am reviewing the ingredients labels and trying to understand what is happening. Again, I wish to be clear about this; there is nothing amiss going on here. DO in this tank never drops below 5 ppm, environmental metabolism is robust, and the BOD is extremely low. I just want to have a more detailed grasp of the chemistry. I am expecting (out of imperfect knowledge to be sure) that in order to identify substances which are reduced, I would typically - but not necessarily exclusively - be searching for organic molecules. I am seeing gluconate, protein hydrolysates (peptides and amino acids), mannitol, ascorbic acid, and polycyclogluteracetyl; and a lot of anions, especially ferrous iron in quantity (which is suggestive). Flourish Trace appears to be all inorganic salts, mostly sulfates, with no complexing agents (chelates), and does not contain any organic compounds.

I think I can generalize a finding that all the named SeaChem Flourish products, taken together, constitute a veritable cocktail of reduced compounds and metals. That adding them to the water column definitely causes the stated reactions in my instrumentation is observed fact.

Thanks for the detailed post, paulg!

This particular paragraph pretty much sums it up and hits the nail on the head as far as what is going on in the aquarium. Essentially, organics will cause a decrease in ORP, as they become oxidized, they will increase chemical oxygen demand, thus decreasing DO and ORP. On the flip side, when DO increases, ORP will also increase. This should answer your question, but if you need further info, just let us know!

[paulg]

I am very grateful for your timely response. The one remaining question concerns Trace. If Trace contains no organics, why would I see an effect on the ORP? Should I repeat that experiment and check whether it is actually doing so?

[Tech Support AN]

No worries!

The concentration of the ions in Trace are low enough to not have significant impact on oxygen in the tank. This is assuming the sulfate isn’t getting oxidized into sulfur (a nutrient plants need in relatively high quantity) when DO is high. Also important to note; a heavy plant load WILL impact DO in the dark period “overnight” as they consume oxygen produced during the light period “day” (BOD). This is the main reason many high level planted keepers aerate their tanks at “night”. Amano started doing this decades ago by raising the filter outlets to increase surface agitation during lights out. Lastly, plants exude electrolytes and other metabolites when their leaf surfaces are compromised. This will impact COD to a degree and is also why we tend to see algae, especially BBA and Staghorn, on plants that aren’t in optimum shape.

I hope this clarifies everything, but should you have further questions, please let us know!

[paulg]

You have confirmed for me that the organics in Flourish, Excel, and Advance are the reason for the oxygen/oxidation dip. I appreciate this because I need to know that I understand it correctly. Just to clarify a couple of things:

The tank, again, is on an automatic aeration scheme. The air pump is controlled by the oxygen monitor. Threshold is 6.0 ppm O2. Likewise the CO2 solenoid is on a pH monitor/controller. That is set for 6.85. These levels are strictly maintained day and night. Photosynthesis brings O2 to saturation (9 ppm) by about 13:00, at least 10 ppm by about 19:00, and then descends after lights out to 6 ppm at about 03:00 when the aeration starts. This is a reliable diurnal DO cycle, and O2 is never under 5.5 ppm in this tank. I have the pump returns adjusted for ripple, not chop; the only reason this matters in my tank is that I avoid blowing off CO2 to the maximum extent possible, not because I need to enhance O2 uptake by this means. Alkalinity is maintained at 7.5 dKH so the CO2 stays in the low to mid 30s ppm.

I reserve the use of the term BOD for the O2 requirement to oxidize DOM (dissolved organic matter), which then is taken as a measure of the concentration of DOM in the system. This is the terminology commonly encountered in the environmental engineering field, a usage with which I am familiar. I don't disagree necessarily with applying its use to refer to plant respiration, but I want to be clear that when I say BOD is low, I mean that DOM concentrations are consistent with unpolluted oligotrophic primary streams and pools.

Trace does not cause an O2 dip, but does create a very small ORP notch. I doubt rapid sulfate oxidation because a daily generous dose of MgSO4 does not have the effect. I am satisfied that this effect of Trace, being so small, is both insignificant and idiopathic/idiosyncratic. As a practical matter, I will rule Trace out as a contributor to the effect.

I keep my plants on pretty good condition, and I have no algae, other than the algal component of the native periphyton (aufwuchs). I haven't had BBA or any nuisance algae for a very long time. This is another discussion, so I'll stop.

I will continue to trust the SeaChem products I am using. Thanks for the feedback.