A good understanding of ammonia is a great basis for getting the most out of any aquaponic system. It is a key element in ensuring healthy fish and plants. Ammonia is highly toxic and consequently needs to be removed. If allowed to accumulate in a fish tank it will certainly cause undue stress, encourage disease and even lead to premature death. It is essential to have a good understanding of the relationship between fish and ammonia for anyone wishing to operate any kind of successful fish culture unit.
What is Ammonia?
Ammonia consists of one atom of nitrogen bound to three atoms of hydrogen (NH3). It is a central player in the Nitrogen Cycle. An understanding of which is fundamental for successful biofiltration. Ammonia is extremely soluble in water. At room temperature 300 litres will dissolve in just 1 litre of water. As a direct consequence of this high solubility, aeration will not drive off this ammonia.
Where does ammonia come from?
There are three main sources of ammonia in a fish tank or aquarium. The first, and most important, is through normal excretion via the gills. Ammonia is the main waste product of protein metabolism in all aquatic animals, freshwater and marine, from protozoa to the most highly evolved.
It is the protein content of the food that is the ultimate source of all the ammonia. Feeding pond fish correctly therefore plays a key role in controlling ammonia production. However,both the quantity and quality of the food will inevitably have a controlling influence over just how much ammonia fish will produce. This direct relationship is really important and offers the best basis for calculating the overall size that a bio filter (grow bed) needs to be in order to satisfactorily control ammonia at safe levels.
The second source of ammonia, often forgotten, is derived from the breakdown of suspended solids. These may be in the form of uneaten food or undigested food contained within the faeces. All nitrogen introduced into a system will ultimately be broken down, by bacteria, through the process of ammonification and add to the accumulation of ammonia.
Ammonification is also a part of the Nitrogen Cycle and requires a significant quantity of oxygen to fuel the process. This is why it is important to flood and drain a grow bed to allow a good flow of oxygen throughout the media.
The third potential source of ammonia worth mentioning is from dead fish. Hopefully there will never be any! If this should occur the subsequent increase in ammonia can quickly threaten the health of the entire fish tank. Make sure that dead fish are easily spotted and always check for their presence whenever an unexpected peak in ammonia has occurred.
Chemistry of Ammonia
It is well worth looking more closely at the chemistry of ammonia as this has a vitally important influence over just how toxic it is. Ammonia is a colourless gas with a strong characteristic odour. It freezes at -77 °C. and boils at -33 °C. at a pressure of 1 atmosphere. At greater pressures it is easily liquified at room temperature. (A factor made use of in refrigeration systems)
When ammonia dissolves in water it forms a weakly basic solution as shown in the equation below.
From the above equation it can be seen that this nitrogenous compound is present in two distinct forms that are in equilibrium with each other. An ionised form, NH4+, more correctly called ammonium ion, and an unionised form, NH3. It is the latter form that is correctly referred to as ammonia.
Unfortunately, ammonia and ammonium are frequently used exchangeably. Ammonia is toxic but ammonium is not. It is important to deferenciate between the two. This toxicity is due to the fact the ammonium ion is unable to cross tissue barriers whereas ammonia is able to do so.
The highest levels of NH3 occur at high pH and high temperatures. These are the conditions when ammonia is at its most toxic.
Of the two variables the pH has the largest effect. For every rise in pH value of 1 , e.g. pH 7.0 to pH 8.0, there is roughly a tenfold increase in the concentration of ammonia. This is hugely influential on the toxicity. By way of an example, fish are able to withstand 100 times as much total ammonia at pH 6.5 than at pH 8.5,
In order to obtain a measure of the toxic NH3 it is necessary to measure the total ammonia present, look up the percentage presence of ammonia on table and carry out a simple calculation. Not particularly straightforward I know, but meaningless otherwise.
Total Ammonia (TAN) = Unionised Ammonia (NH3) + Ionised Ammonia (NH4)
|6 °C||10 °C||14 °C||18 °C||22 °C||26 °C|
Table 1 : Percent Unionised Ammonia at varying pH and Temperature
How to Measure Ammonia
Test kits are readily available and relatively inexpensive. For most people these kits are ideally suitable. They are quick and simple to use and a zero reading provides reassurance that everything is fine. However, be aware that a positive result (one indicating the presence of ammonia) ay not be as bad as the kit suggests. Remember to use Table 1 above to calculate the toxic ammonia present rather than the total ammonia. Some people have used a ‘safe’ ammonia level and back calculated the amount of total ammonia that represents this safe level. These are interesting and can provide reassurance. I still prefer to calculate the actual ammonia present and judge just how safe this is by the way the fish are feeding and behaving.
If optimising the performance of your aquaponics system is important to use a more accurate analysis than a drop kit. A dedicated meter may well be of interest. Certainly any commercial unit would be better off with a dedicated photometer. These units are also easy to use but give an accuracy better than 0.05mg/l. I cannot recommend these units highly enough as they are indispensable when assessing a filters performance.
Exactly where and how many samples are taken should also be considered. I would recommend commercial units analyse at least two sample points at the same time. One entering the bio filter, and one leaving it. (This often equates to one entering and one leaving the fish tank) This will provide a great deal of really useful information about how well a filter is performing and how much ammonia the fish are producing.
Aquacultural scientists invariably report figures across a filter. Koi keepers, on the other hand, rarely do. The latter may be surprised to know that commercial fish tanks, and probably most koi ponds, never operate at zero levels of ammonia. Also, ammonia levels leaving a koi filter are quite often higher than those going in. What does that say about filter performance? Even when there is an expected drop across a filter it never drops as far as zero.
Reporting ammonia levels
It is also worth understanding how ammonia levels are best reported as this can, and does, cause widespread confusion. Test kits will usually analyse the nitrogen component of the ammonia and ammonium ion combined to give a figure for Total Ammoniacal Nitrogen (TAN). I much prefer this method of reporting as it clarifies the state of the ammonia ands implies the chemical equations involved as it bases the relationships on the N atoms involved. All too often authors talk about ammonia when they really mean TAN. As we reported in Table 1 the former is usually only a small and variable fraction of the latter. Aquaculture has adopted TAN but is far less used by koi keepers.
So what is safe?
It is often stated that the only acceptable safe level of ammonia is zero. Whilst it is hard to disagree it is inevitable that some ammonia will always be present in the fish tank before it is passed to the bio filter. Indeed, if no ammonia is present then the bacteria will no longer have a food source and would die.
Simple test kits often lack the precision to detect such low levels so it is understandable why so many pond keepers report a zero level in their ponds. In order to assess the performance of a bio filter a methiod of analysis that has an accuracy of plus or minus 0.05 mg/l of TAN is highly recommended.
Deciding on a safe limit is not an easy thing to do. Scientists have used a technique to compare toxicity of one compound to another by determine what concentration (or oral dose) will kill 50% of the population within a set period of time, often 24, 48 or 96 hours. Not a pleasant procedure but one that, at least, does allow us to test how different conditions can affect toxicity. As part of my degree project I found the LC50 for ammonia to vary with temperature as shown in Table 2.
|Temperature||48 hour LC50. mg/l NH3-N|
Table 2 : Effect of Temperature on Ammonia Toxicity in Carp
It seems reasonable to conclude that carp are more tolerant of ammonia the closer they are to their optimum temperature.
Species vary significantly as to how tolerant they are to ammonia. Generally, fish that live in colder water, such as salmon and trout, are less tolerant than warm water species such as carp, tilapia and catfish. Also marine species are less tolerant than freshwater species.
The determination of the LC50 is hardly a sound basis for setting a safe limit. Fish must come to no harm because the ammonia is too high. Setting the level as a percentage of LC50 is also not really advisable. Stress, as well as morphological changes, need to be considered. Safe levels cannot be set in stone. Tables that specify exact safe levels should be interpreted with care. Remember, measure accurately, follow trends with a graph and compare any changes in fish behaviour or incidence of disease with any change in water quality.
At the end of the day a single figure for the safe , but maximum, level of ammonia is useful as a guideline. That recommended by the OATA is probably as good a guide as any. As a result their figures have been widely adopted. They recommend a maximum safe level of no more than 0.02 mg/l NH3 for freshwater fish.
However, bear in mind that this level could be present with a TAN of over 100mg/l if the pH is 6.0 and the temperature 10 C. Way beyond any recommended level of TAN. I myself have witnessed levels of TAN above 100mg/l NH3 under similar conditions following a pH crash with no incidence of disease or mortality. Definitely not to be recommended though. Control of alkalinity is the recommended method of preventing this from happening.