Continued from page 1My contention is that when a mass of water sits underneath these layers and that it is so high in nutrients, as documented in the article on the tests conducted, the potential for large amounts of nutrients getting back into the tank is high, as the difference between the tank parameters themselves and the plenum water parameters are of more than just incidental differences. This is documented in the article.
In addition, the fact that other tanks with plenums were showing signs of lower water quality and low redox levels, difficulty in managing the pH, sporadic outbreaks of diatoms and lower green algae, makes me suggest that the potential for problems exists when the plenum is so loaded with nutrients. This is especially so if the manner in which the plenum is set up is conducive to releasing these nutrients back into the tank either through osmosis or rediffusion (balance seeking between the two layers).
In reply to Raymond's concern, it is indeed correct that these nutrients in the plenum are not manufactured there, rather they concentrate in that water mass over time, and originate from the water in the tank and from the metabolic activity going on there.
The biochemical activity going on in the sand layers and in the layer below, appear to digest organic matter and diffuse the mineralization by products into the plenum water (there really is not other explanation for the fact that nutrient amounts in the plenum water rise so high unless this is the process that takes place).
The speed at which this build up occurs and the decline in the water quality of the tank, are related to the bioload in the aquarium. The higher the load of animals, the faster these negative effects become visible.
Again, all these observations and the sad condition of some of the animals observed in other tanks is what eventually led to the thorough testing done on the plenum of the tank described in the other article referred to already.
In tanks without a plenum, such metabolic processes occur as well of course. The difference observed was that no depressed water quality conditions came about and that the nutrient base which was lowering all parameters used to gauge water quality remained stable after the initial run-in or cycle period.
Obviously since there was no area for them to concentrate in, they were gradually being removed by the normal filtration techniques used on the aquarium, and did not transfer to the water in the plenum.
The plenum, in essence, "absorbed" these nutrients, making it impossible for the filtration system to remove them from the aquarium. The biochemical activity that brings this removal about in tanks without a plenum must therefore be concentrated in the live rock that is present in the tank, in combination with the beneficial effects of using a protein skimmer.
The main reason for this statement is that those are the only two major gateways for these nutrients to be removed. To some degree compounds used in the aquarium to lower nitrate, phosphate and silicate enter the picture as well, but to a lesser degree since no large quantities were used in such aquariums.
In the large tank, the water quality never demonstrated rapid or large rises of any of these. The fact remains though that they were always present in small quantities regardless of how much compound was used to try to remove them.
The large amounts in the plenum water replenished their concentrations on a steady basis, by leaching or osmosis, as explained, on a continuous basis, depressing the water quality to the point where the animals visibly suffered from this low but constant nutrient presence level (and the lower oxygen and ORP indicated).
The only way to keep these other smaller systems in balance was to skim at very high levels and carefully monitor conditions, and intervene when necessary.
Interventions included using all the standard methods that hobbyists resort to when such a situation occurs. This is detailed more later in this article and the companion one.
As was pointed out in another message, compounds to remove phosphate and silicate can be used and that became necessary in that tank.
When a choice needs to be made between a system where this danger does not exist (no plenum) and one where it does (with plenum), common sense make me lean towards the system where this danger does not exist.
Raymond points out that he has not heard many complaints from others using tanks with plenums. Perhaps this is due to the fact that no study has been undertaken to compare results in both type of set-ups and that no feedback has been requested.
Maybe hobbyists who have tanks with a plenum are not realizing that their corals could look a lot better, or they are using the remedial techniques to combat the depressed water conditions. This leads to a far greater expense of course as they need to use larger amounts of nutrient removing compounds and perform more frequent and larger water changes.
At least one other hobbyist has indicated that he has set up a mechanism that very slowly takes the water out of the plenum so new water from the tank can trickle into it, to prevent the build up of nutrients referred to.
Where do the nutrients go when no plenum is present? They are digested by the bacteria in the sand and otherwise removed by skimming, water changes, biochemical activity in the tank and in the sand and rocks.
When the plenum is present some may be removed in the same manner but the fact is that some are not are end up increasing in the plenum water.
Is there a benefit to this accumulation? I cannot see one (I read the description of using the layer to increase calcium by removing water and dripping it slowly back into the tank etc. - that is not a true plenum system, though but again a further modification, as the water in the plenum is not left alone but constantly replaced by this process and, as a result, this constitutes a different method yet).
A message was posted that there actually was a benefit to this build up of nutrients and that Bob Goemans has written an article to this effect. As I have not read Bob Goemans' article I cannot make any judgment as to the what Bob has to say and how he explains what the benefits are. Will have to wait for the article itself.
The system tested was set up in such a manner that it conformed to the classic set up of a plenum system with the edges of the grid sealed against the glass. The only manner in which nutrients could get back into the water was by seeping through the sand layers.
I did not notice a reduction in need for Kalkwasser (KW) addition as pointed out in another message. I had to continuously drip KW into the tank. The fact that calcium levels remain easier to maintain, as one hobbyist pointed out, may not have to do with what is in the plenum itself, but with how the layer meant to assist calcium maintenance is made up. Depending on the material used in that layer, calcium levels may indeed be "fed" from that layer for some time.
When that layer starts to coat itself with magnesium ions though, and starts cementing itself (forming clumps) this activity would stop as there would no longer be a leaching of calcium into the water.
In aquariums with the sand right on the bottom of the tank, I have not run into any outbreaks of algae or diatoms and have not had lowered ORP levels.
Whatever differentiates the chemistry of the two systems (with plenum and without) is to my knowledge not documented.
How the nutrient recycling occurs is not entirely clear, at least not from my experience and what I have read and seen.
What I do know is that aquariums without a plenum operate without these difficulties and appear to be far easier to manage.
Note that in all systems described, the plenum grid edges were tightly sealed to the glass. Leaching did, therefore, not occur at the interface between the grid and the glass. If it did not occur in that manner, the only other way for it to make its way into the tank's water was through osmosis across the sand layers.
The actual plenum in the large tank (from the bottom of the tank to the bottom of the eggcrate) was 2.5 inches of water.
As of now, given all of the above, I lean towards not installing plenums and prefer to place the sand right on the bottom of the tank.
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