Hi all,
So inside the biofilm deeper in the media, the facultative bacteria living there will not get any free O. But instead of dying off to be in balance with the available O for them, they are able to switch over to using nitrate. When they do this they are no longer using free O. since it has been used up by the other bacteria. However, now they are denitrifying the tank, which is a benefit. So, bacteria that were using free O stop doing so and nitrate which was accumulating is now being reduced.
I think we will just have to differ on whether denitrification in the filter material is a good idea. I'm not saying it
isn't possible, because it obviously is, but I'm really interested in the probable, rather than the possible.
Risk management, negative feedback and single point of failure
I'm going to talk about
"single point of failure", resilience and risk assessment. They are fairly prosaic titles, and it isn't going to be a very exciting, but I would recommend it as an approach to everybody.
Single point of failure
This all becomes much more relevant in those millions of tanks which have no plants and/or no substrate. It is fine to say "I do only planted tanks." unfortunately that is of little help to all those tanks with no or minimal plants. "Help, I have 40 ppm nitrate in my tap water and I keep an overstocked African rift lake tank for Tangs. How can I deal with high nitrate?"
Problems with this is the filter has now become a
"single point of failure".
It is also a severe risk and has a reasonable likelihood of happening.
If I was in this position, I would want the "
no substrate, no plants" scenario to occur for as short a time as possible. As soon as I could I would add a substrate, even a thin layer of sand. I've now added resilience (I'm still dependent upon the filter, but I have a secondary site for nitrification in the substrate). If I add plants (they can be planted in a separate tank, or in planting basket with their stems emergent, or in an over-tank trickle filter) I've added more resilience, and also a negative feedback loop and an indication of the nutrient status of the tank (via the
Duckweed Index <
http://www.planetcatfish.com/forum/view ... weed+index>).
Tank design and management based upon risk assessment
It's not a "sexy" subject, but I'll cover it anyway.
My primary concern, and the primary requirement of all fish is that the water has enough dissolved oxygen at all times. If we have even a very limited period where the water doesn't have enough oxygen for the fishes requirement, the fish will die. Oxygen requirement will differ between fish, but large rheophilic fish are particularly at risk, which is partially why I wrote "
Aeration and dissolved oxygen in the aquarium" <
http://plecoplanet.com/?page_id=829>, this covers the mechanisms that deplete, and replenish, oxygen in the aquarium. As well as tank dimensions, and methods of adding oxygen to the water and removing CO2 etc., it has quite a lot on biological filtration because that is one of the major users of dissolved oxygen.
Because low oxygen levels are a severe risk I need to minimise the chances of them occurring, which is why I don't want potential single points of failure, or systems which require continual maintenance to keep them functioning. High NO3 levels are a low risk to the fish, but high NH3 levels will kill them by both direct toxicity and by depleting the available dissolved oxygen.
I want stable resilient systems, where if things go wrong I have secondary systems (an additional filter etc.) and negative feedback loops ("belt and braces") to keep the fish alive, and buy me enough time to make the necessary changes.
cheers Darrel