Hi all,
I have seen on the forums and YouTube etc all manor of weird and wonderful setups lol.
I know this is a forum, but most of what is written on forums about filtration is wrong on all sorts of levels.
The filter bit
You want any mechanical filtration to occur before the water enters the biological filtration, ideally outside of the canister filter. Like the others have said plants are very efficient at removing fixed nitrogen from the water column, and you just need to ensure that water flow through the filter isn't impeded. This means no fine sponge, or floss, inside the filter.
The issues with Canister filters
Canister filters are convenient to use, but their efficiency is limited by their design, where the oxygen that enters the filter can't be replenished while it is in the sealed filter body. Nitrification is an oxygen intensive process (we've gone from NH3 to NO3) and the real issue is that if the oxygen levels fall too far aerobic nitrification is compromised and the levels of ammonia (NH3) and nitrite (NO2) in the aquarium start to rise. Other filter types (HMFs, HOBs, Trickle filters) have access to dissolved oxygen during biofiltration which make oxygen issues much less likely to occur.
Ammonia and nitrite are toxic to fish and shrimps at low levels, but nitrate isn't toxic until we reach levels in the hundreds of ppm range. Nitrate is the "smoking gun" of previous high levels of nitrite and ammonia, but it isn't the bullet.
Nitrate can be removed from water in a number of ways, these include <"
chemical ion exchange resins"> (where a Cl- ion is swapped for a NO3- ion), water changes with water lower in nitrate, <"
anaerobic denitrification"> where bacteria use NO3 as their oxygen source and nitrogen (N2) gas is produced and
<"incorporation into plant tissues"> as the building blocks of photosynthetic proteins.
A lot of the discussion on filtration looks upon the complete simultaneous nitrification/denitrification arc within a canister filter as a good idea, but it definitely isn't. The reason for this goes back to the filter design and the fixed amount of oxygen that enters the filter.
The sellers of Biohome etc. will tell you that the porous nature of their media allows aerobic nitrification to occur in the outer layer of bacteria, which then deplete the oxygen sufficiently for simultaneous denitrification in the inner micro-pores. This can occur, but because of the very small spatial separation between processes you need a steep oxygen gradient across the biofilm. To achieve this often we have to slow flow rates down until we reach a sweet spot where the bioload (BOD in the schematic) and oxygen supply are balanced.
The problem is that if you don't have sufficient oxygen entering the filter you run the real risk that nitrification becomes compromised, in fact I would go as far as to say that nitrification will always eventually become compromised in this scenario by a further slowing of the flow, or increase in the bioload (either the gradual process of fish growth, or a traumatic event like fish death or emergency chloramine dosing etc).
If you aim for a high flow rate of water, that is initially fully oxygenated, then you have a much more robust system, where you are never going to have high levels of ammonia or nitrite. Heavily planted aquariums, with a substrate, have a much larger capacity to treat bioload, because "plant/microbe" filtration is much more efficient than "microbe only" filtration, and plants are <"
net oxygen producers">.
cheers Darrel
