Today - I did a couple of analyses of my water below my DSB.
There has been a lot of discussions lately if there can be any total ammonia "pockets" in an existing mature tank. My tank is constructed in a way that it is rather easy to investigate this. It has a DSB in the refugium that have a reversed flow. I pump water down to the plenum and further up through the DSB
The flow is around 1000 L/H through this compartment of my aquarium with water containing around 0.1 NH3/NH4 as I show below (from march).
My PO4 is rather high and on its rising - for the moment 0.89 in the display water. 1 week ago - I had to switch my pump that supplies the plenum with water from the display. The same type but it seems to give what is supposed to do (around 120 L/H) -> 2880 L/24 hour. The old gave maybe only half that amount as it was old and tired. I have gone down with flow now to around 58 L/H.
What did I find and am I surprised? I´m not surprised by myself - it was in line with what I had expected due to experiences in both anaerobic and aerobic environments I have been working with before but it maybe will surprise some others - like those that always claim that tanks with a lot of corals and surface never can produce high amounts of NH3/NH4 and therefore all measurements that shows the opposite is false.
In clear figures
NH3/NH4 (total ammonia) in the DT - around 0.07 mg/L
NH3/NH4 (total ammonia) in the bottom of the DSB - around 0.89
PO4 in DT - around 0.89 mg/L
PO4 in the bottom of the DSB - around 1.78 (read 0.89 too but it was diluted fifty/fifty with new fresh saltwater)
NO3 in the DT - around 6.4
NO3 in the bottom of the DSB - around 3,8 mg/L
NO2 in the DT - around 0.013 mg/L
NO2 in the bottom of the DSB - around 0.03
These figures shows that activity of heterotrophic bacteria produce a lot of both NH3/NH4 and PO4 when they are feeded with organic carbon. It also shows that for the moment - my DSB mostly act as a NO3 producer (its probably mostly aerobic) and I have slowing down the flow and increase the organic carbon dosing in order to favour denitrification. The nitrification will happens at other places in my tank.
Sincerely Lasse