Fish ammonia, if allowed to accumulate in a tank, can quickly rise to lethal levels. The best method of preventing this from happening is to cultivate a healthy population of filter bacteria. Ammonia oxidising bacteria, AOB, also know as nitrifiers, will feed on this ammonia and convert it into far less toxic nitrate. The latter constitutes an important source of plant nutrients. Keeping nitrifiers happy is therefore an important step for the health of our fish and the nutrition of our plants.
These are the conditions under which nitrifiers will thrive.
1. Reduce the competition from heterotrophic bacteria.
The nitrifiers are the good guys. One cell can convert as much ammonia as one million heterotrophs. Unfortunately, they have evolved this efficiency because they are very sow to reproduce. It takes 15 – 25 hours to double a population of ammonia oxidisers, even under optimal conditions. Heterotrophs, on the other hand can accomplish this in just 15 – 60 minutes. In the time it takes Nitrosomonas (a principal nitrifies) to increase from one to two cells something like E.coli could potentially number 35 trillion cells. As a direct consequence of this slow growth potential they are extremely vulnerable to competition from heterotrophic bacteria. Nitrite oxidisers have an even slower doubling rate of around 60 hours.
Heterotrophic bacteria not only smother and compete with nitrifiers they may also prey upon them.
Undoubtably the best way to reduce this competition is to reduce the food source of the bad guys by removing as much organic matter as possible. Aquaculture systems utilise settling, trapping and screening techniques to achieve this. In aquaponics the removal of solids is less vital as mineralisation of fish poo provides the plants with nutrients. Stocking rates however need to be lower to allow proper zonation of heterotrophic and AOB to occur within the grow beds.
2. Maintain a high temperature.
The optimum temperature for Nitrosomonas is 25 – 30 C. Nitrite reducing bacteria also grow best at warm temperatures. As the temperature drops then so does the rate at which ammonia can be consumed. At 4 C. nitrification all but stops and will die at 0 C. Nitrobacter is less tolerant of low temperatures than Nitrosomonas.
3.Employ a high surface area media in your grow bed.
Nitrifiers are largely non-motile and actively seek out a surface to attach themselves to. They will secure themselves by producing a biofilm. The more space you give your nitrifiers the higher the population you can maintain and the more ammonia that can be controlled. However, the surface area must be attractive to the good rather than the bad guys. If the media accumulates sludge and restricts water flow it s far from beneficial.
The simplest way of increasing surface area is to build a bigger grow bed. Always tend to oversize a grow bed in relation to the number of fish rather than undersize.
4. Maintain adequate levels of dissolved oxygen.
Nitrifiers are strictly aerobic autotrophs and the oxidation of ammonia clearly involves oxygen. Like fish they thrive best at dissolved oxygen levels that exceed 60% air saturation. When levels drop below 3mg/l. nitrification virtually stops. The chemical reaction involved is shown below.
NH3 + 1.5 O2 ———> NO2 + H2O + H+
Every kg. of ammonia requires 4.5 kg. of oxygen. Whereas this is well below that required to ultimately breakdown fish poo it is significant. There has been a steady move towards aerated biofilters in aquaculture. The flood and drain aquaponic techniques achieves the same result.
5. Maintain a high water flow rate.
Nitrifiers are very efficient at converting ammonia so they do not require much in the way of retention time. However, I have never come across a biofilter that removes 100% of the ammonia that enters it. Rates of removal are typically 20 – 40% per pass. This will depend upon the flow rate and hence retention time in the grow bed. A slow rate increases the retention time and increases the % drop across the grow bed. However, the total ammonia breakdown per day is normally lower because of the lower flow rates. By maintaining a faster flow rate the background level in the fish tank will be lower. It is therefore advisable to err on the side of faster flow rates than longer retention times. If you experience ammonia spikes is always worth trying to increase water flow rates.
A general rule of thumb is to exchange the entire fish tank volume every hour. I wouldn’t hesitate to increase to every 30 or even 20 minutes if fish stocking rates are high. I have operated a 100m3 system with a 6 minute retention with no ill effects other than wasting electricity!
6. Control pH and alkalinity.
Nitrifiers, in common with all life, require a source of carbon to produce more cells. They are able to utilise free carbon dioxide as their carbon source. In so doing they generate free acid and hence nitrification will result in a fall in pH. This acid reacts with the bicarbonate (HCO3-) to release more carbon dioxide (CO2). More than that consumed. Approxiamately 5.9 mg/l of CO2 is generated for every 1mg/l of TAN broken down.
The optimum pH of Nitrosomonas is widely reported as 7.8 -8.0. The optimum of Nitrobacter is slightly lower at 7.3 – 7.5. These are above the generally accepted ideal pH for plant cultivation that is around 6.5 – 7.0. There is no point therefore in trying to maintain the fish and grow beds at the higher temperature. It is a comfort however that should the pH rise, leading to an increase in fish ammonia toxicity, the nitrifiers should be able to work a little but harder.
7. Self cleaning grow beds
Even if you employ a solid removal system they are rarely 100% effective. A certain percentage will always escape the removal system and accumulate within the grow bed. In addition the water conditions are also ideal for many types of bacteria to flourish, both good and bad. These will themselves grow, reproduce and die to add to this accumulation. This occurs throughout the system, including the pipework.
Some sort of self-cleaning process is usually beneficial. The flood and drain technique achieves this quite successfully. It does have limitations however and if stocking rates and feed rates are high then the need for self cleaning biofilters to breakdown the ammonia increases.
In aquaculture systems this is normally achieved by keeping the media in constant motion by injecting air into the filter media and maintaining a high water flow. A trickle filter is more akin to a flood and drain grow bed as the media is kept wet but not flooded to allow solids to accumulate. A rotating biological contractor tackles the problem by constantly rotating the filter, that is part submerged in the water column. Both systems have the added advantage of providing the necessary oxygen required for nitrification.
Nitrifying bacteria are photo sensitive and grow best in dark conditions. (Vergara et al, 2016 ) They also found that nitrite oxidisers were more affected by light than AOB.
Keeping the light levels low also helps prevent excessive algal growth.