CONCLUSIONS DRAWN FROM THE SWCR EXPERIMENT
After several days of testing zero NO3, but normal PO4, I have a concern about nitrogen insufficiency. Certainly DIN is being continuously evolved, but between more sparing feeding and increase in SWCR rate, there is no surplus water column NO3. To the extent that nitrogen is bioavailable from all sources, the plants are consuming it. Last night and again this morning, 15 mL KNO3 w/urea (SeaChem Nitrogen), was added. In the newly accelerated SWCR, a dose will be calibrated to assure non-zero NO3. Something learned in the past few months of trials in this water change scheme is that maintaining non-zero nitrate derived solely from fish food can result in DOM levels that are difficult to manage, even with liberal application of chemical filtration media. Though attractive is the idea of gaining growth factor nutrition from natural outfall of environmental metabolism, it does not work so well in what is essentially a closed system due to unavoidable concomitant generation of dissolved organics. This accentuates the superiority of water change over chemical filtration for pollution control in closed systems. It is preferable to resort to pure supplementary NO3 in correctly calibrated dosage and to double down on all DOM reducing strategies, in particular water changes. It is a mistake to infer low DOM from low DIN and low orthophosphate. True oligotrophy is defined by diminished figures for all three of these parameters. The problem arises from the relative difficulty in reliably measuring DOM. Redox potential should be a fairly good indicator, but my data shows that there is more to it. I am convinced that DOM is a primary growth factor in algae population. Incipient nuisance algae is an indicator of high DOM, and the relationship of this condition to ORP is not so straightforward as I had expected. A highly oxidizing environment certainly will derive from persistent high oxygen tension, but this does not necessarily ensure that DOM is not also present in larger than expected concentrations. I have been puzzling over the gradual eutrophication of this system while seeing very high ORP and very low DIN. Overfeeding will eventually manifest and cannot - cannot - be corrected with activated carbon, regardless of the charge weight.
The SWCR does work, and is best at high change rates. As long as various means of supplementation are implemented to provide stable background chemistry and sufficient plant nutrient, the faster the turnover the more efficiently does the system shed DOM. The Hardness Reconstitution Reactor (HRR) and K2CO3/MgSO4 dosing can be adjusted with relative ease to attain K+, Ca++, Mg++, and CO3-- at appropriate concentrations. This strategy for maintaining a planted tank indefinitely through automated techniques is highly promising. Just now it looks like fish food provided sanely will support non-zero orthophosphate within oligotrophy. Supplementing with KNO3 to satisfy nitrogen requirement is a simple matter.
Today's numbers
NO3: 0.89 ppm
PO4: 0.12 ppm
K: 15 ppm
dKH: 5.60
dGH: 3.59
Ca: 17.6 ppm
Mg: 4.88 ppm
Potassium and alkalinity falling behind the SWCR. I am increasing K2CO3 to 120 seconds per dose, 20 mL three times per day (= 60 mL daily total).
Magnesium also a bit short. I am increasing MgSO4 to 120 seconds per dose, 20 mL twice a day (= 40 mL daily total).
I am restarting KNO3 w/urea (SeaChem Nitrogen) at 5 mL (30 seconds) once daily.
All subject to revision as the situation develops.
Pretty clearly, the increased SWCR rate is effectively diluting the water column. The reduction in DOM is having an immediate impact. Confidence is high that the lighting can be increased, as is the plan, without occasioning onset of excessive algal growth. It has taken just a couple of days for this to prove out, as much of the scuzzy periphyton has spontaneously abated, as has the stubborn recurring oil slick.
Consideration of effects to micronutrient/trace is due. At present I am not compelled to increase doses here because I have no data, but this will not be neglected.
