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[APD] Some thoughts on substrates and some Heat cable debate
Some thoughts on substrates:
In reviewing some of the discussion that Dupla espouses regarding the benefits of cables I am having a difficult time understanding any benefit. It would seem based on their logic that adding additional __flow__ of any kind would be detrimental, not benefitial.
For redox reactions to occur as they suggest the cable help, you need less flow and a lower Redox value than cables would provide even if you took the lowest possible values that they would provide(say 1 C temp thremal difference between the water column and the bottom of the gravel substrate layer.
Fe reductions occur in soil substrates in wetland around 200-100 mv range. Given the porosity of the grain size in aquariums, simple diffusional gradients alone(eg no heat cables, simple diffusion alone) are too much flow through the substrate to achieve this.
There are differences in grain size and very importantly __organic material added__(hence lower redox values with more organic matter) to individual substrates. So one will expect that two substrates identically set up one with say peat added(or mulm, established bacteria etc) vs one with no peat added, would have much better redox conditions more condusive for this range of Eh mv range to occur(the one with bacteria and peat).
The organic material added is very important here.
If you add an old rotten Aponotgeton bulb, deep in the substrate, it has enough(too much) organic material and this causes sulfur reduction to occur and causes very low redox values, what we generally want is moderate low redox values compared to the water column which ideally will have very high levels(due to all the O2 from the plant production), not extremes.
More flow would bring the high O2 rich water into these areas and raise the redox values, not lower them. This might be good if you have a lot/too much organic matter. Lots of Fe(which would buffer the lowering of the Redox level) and NO2 will also help from lowering ther Redox down too low.
You need a balance of organic matter, Fe etc to provide a good Redox value in your substrate, not too low, not too high.
Black and white explanations of having a "reducing environment" is simple but too simple. A balanced Redox value is what we want for good growth, not "more reduction is better".
I just do not think nor believe that cables can produce the magic optimal flow rates.
I think the elevated temps help bacteria cycle the nutrient better, brings more O2 for aerobic degradation of oragnic matter which occurs at a much faster rate than anaerobic degradation.
The export of "waste"?
Waste is the break down of organic matter and this seems like it would be of great benefit to the plants, not something an aquarist would want removed from the tank's substrate. The plants remove waste from the breakdown of organic matter in the form of CO2 uptake from the substrate, NH4+, PO4 etc.
Water changes will remove most waste and cause a diffusion gradient that would dilute and transport waste out of the substrate just fine. I suppose some might believe in allelopathic root exuded compounds may cause significant issues and the cables may help remove these compunds but I think the flow rates without cables is enough to not have these issues become large enough to cause problems. Another idea is that if root density is high, then flow rates in/out of the substrate in these areas is also going to be high and O2 levels will also be high in these areas. Roots will pump considerable amounts of exudants and gases in/out.
Excess waste in the substrate can lead to too much organic loading and cause the substrate to reduce sulfur and this is around the range of -200 to - 300 mv, this is often found around Aponogeton bulbs that have died. But you need a source of organic matter for this reduction to go this far in order to cause problems.
Roots are very active and greatly influence transport of material in/out of the substrate. Bacterial, fungal process also greatly influence and cycle nutrients/waste in out of the substrate.
Established aquariums will process organic matter much faster(tight coupling between waste and re-minerlization=> nutrients for plant uptake) than tanks without good root development, bacteria/fungal cultures.
What's this all mean?
It seems that normal diffusion alone is more than adequate for flow rates and environmental concerns with the substrate in planted aquariums.
Use mulm, perhaps some peat, have deep substrates, these help provide better redox and cycling(better coupling between waste(Carbon, PO4, N etc) production-reminerlization of waste)
Flow rates are too high from cables to provide optimal redox values until much later in the substrates development but can establish a good aerobic environment like a RFUG filter does. After clogging up somewhat, the substrate has enough organic matter to provide better redox values over time, but so do non cable tanks........
So we do not see a great difference in cable vs non cable tanks.
Transport of excess waste in any substrate is not a problem in any tank I know of.
Soil tanks do quite well, these have very low redox values vs a sand laterite Dupla style set up.
RFUG's also do well.
Roots greatly influence redox values in substrates, teasing apart the effects of bacteria/organic matter and redox with and without plants can be tricky. How much better is the the nutrient status for the plants when they need a fair amount of O2 for the roots to respire? Is this low redox worth the cost associated with the lack of O2 available for the roots?
Perhaps but it does not appear to be a large issue for plants.
What to do about this:
I think one thing that gives a good environment for roots and nutrient availabilty is that of Flourite or other porous iron rich substrates(Turface etc).
The internal spaces remain anaerobic and hence low redox values exist even if you disturb the layering and uproot plants. The integrity of the aerobic and the anarobic zones are maintained, and the general profile of the substrate remains aerobic allowing plenty of O2 for root respiration, microbial cycling(aerobic cycling occurs much faster than anaerobic cycling !!!!!!) is much more efficient.
We are not after SO4 reduction, NO3 denitrification, NH4+ sources(algae sources after uprooting/replanting etc) etc so we really do not need anything except some Fe reduction in small zones for the roots
I can certainly say that replanting has much less impact on a RFUG tank than a tank with no flow and it is very likely due to the greater presence of reduced nutrients, likely NH4.
So the micro envirnment inside each porous grain gives the best of both worlds, the benefits of anaerobic and aerobic conditions with out many of the associated potential problems with a layered system.
But I think if one wants to look into this area more, you need to have a substrate with very little in it and gauge that against systems like soil tanks, or flourite etc, add things like peat, mulm etc.
See how the tank does on a general leve(Plants grow better?) and also look into how adding some of these things effects the redox values of the system.
You might not find exact solutions but you can at least come up with some generalizations and ideas that can tell why a substrate works well or not.
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