I started measuring the parameters of the water in the plenum after the sand was in the aquarium for one month. I then took measurements once a month at 30 days interval. The table below shows the results at timeframe 60, 90, 120, 150 and 180 days. I then skipped a few months in the reporting below (although the actual testing was done) and give you the results for the last months of the testing.
The method used for this was to insert a real small pipette through the sand layer and screening to the bottom of the tank so its end was definitely in the water that constituted the plenum. The glass pipette was left in place so no "holes" would be made (interestingly enough, it did not take long for that pipette, placed in the left corner of the tank, to be totally covered with coralline algae).
Below are the results for NO3, PO4, SiO2, pH and and O2. I did not measure any other parameters.
"the data below is based on 30 day interval reading from the plenum"
NO3 PO4 SiO2 pH O2
11 0.7 0.2 7.9 5.4
34 1.8 0.9 7.2 4.9
71 4.1 3.7 6.4 4.2
117 8.1 5.4 6.0 3.9
243 14.7 7.1 5.4 3.7
skipped a few months
for reporting purposes
_
520 19.4 11.3 4.9 3.2
614 21.6 14.7 4.9 3.2
803 23.4 15.3 4.7 3.2
It would appear that the progression of nutrients is gradual and upwards and that oxygen falls and so does the pH.
When I measured the pH after 18 months of running the tank and before taking the plenum out (that was an exercise!) the pH was actually 3.9 and the oxygen level had fallen to below 3mg/l. At that time NO3 was close to 900 ppm.
Of course to take all these test I had to use dilutions in certain cases as none of the tests measured in such high ranges. I remultiplied my results by the dilution factors to arrive at the end results.
Based on this test and a test on one small tank, for a shorter period of time where I discerned the same trends, the conclusion to be drawn from this in my opinion, that plenums pose a potential problem for the hobbyist and the animals in the tank.
Indeed:
These nutrients can leach back into the aquarium by means of osmosis
Any break or rupture or shallow area of the layers could result in these nutrients seeping back through into the water
If this happens to a large extent, the tank is going to be in real bad shape as large amounts of all the nutrients will suddenly mix with the tank's water.
If a slow osmosis of nutrients takes place, the tank will go through unexplained problems and algae outbreasks that cannot really be accounted for. Alternatively or in addition nitrate, phosphate and silicate may show up in the tank that the hobbyist cannot explain in terms of origin. Of course no one would suspect that the build up actually comes from nutrients that leach back into the tank's water from the cesspool of nutrients that the plenum has become.
For this reason I no longer use Plenums in reef tanks and no longer recommend them.
With diligent funnel-syphoning & stirring-up the dsb, I have been able to keep exportation of waste in check despite a heavy bioload in which I feed the reef fish 3X's/d & maintain over 50 SPS's. I have found that doing this once/wk is necessary for my bioload. I personally like the plenum for denitrification & an marker for determining the depth of my bed cleaning[I NEVER disturb the plenum]. Idrhawke's system should work & may end up taking less maintenance. It takes me about 45min/wk. to clean 1/3 of dsb--just to give you all an idea of how easy it is to care for a p/dsb
] gave those suggestions? Bob
Bob
