WATER QUALITY FOR FISH
Nitrogen
Ammonia and nitrite are extremely toxic to fish, and sometimes referred to as “invisible assassins”. Ammonia and nitrite are both considered toxic above levels of 1 mg/litre, although any level of these compounds contributes to fish stress and adverse health effects. There should be close to zero detectable levels of both of these in a seasoned aquaponic system. The biofilter is entirely responsible for transforming these toxic chemicals into a less toxic form. Any detectable levels indicate that the system is unbalanced with an undersized biofilter or that the biofilter is not functioning properly. Ammonia is more toxic in warm basic conditions; if the pH is high, any detectable amount of ammonia is especially dangerous. Water tests for ammonia are called total ammonia nitrogen (TAN), and test for both types of ammonia (ionized and un-ionized). Symptoms of ammonia and nitrite poisoning are often seen as red streaking on the fish body, gills and eyes, scraping on the sides of the tank, gasping at the surface for air, lethargy and death. Nitrate on the other hand is much less toxic to most fish. Most species are able to tolerate levels of more than 400 mg/litre.
Ph
Fish can tolerate a fairly wide range of pH, but do best at levels of
6.5–8.5. Substantial changes in pH in short periods (changes of
0.3 within a period of 12–24 hours) can be problematic or even lethal
for fish. Therefore, it is important to keep the pH as stable as
possible. Buffering with carbonate is recommended to prevent large pH
swings.
Dissolved Oxygen
Overall, as much DO as possible should be added to the aquaponic system.
In practice, most fish require 4–5 mg/litre. Most domestic growers do
not have the ability to check the oxygen level in their units because
digital oxygen meters are expensive and cheaper aquarium test kits are
not widely available. Even so, following these recommendations ensures
adequate DO levels. Do not overstock the fish, and refrain from adding
more than 20 kg of fish per 1 000 litres of total water. Dynamic water
flow, with cascading water falling back into the system, helps to aerate
the water and add DO. Air pumps, if at all feasible, should be used.
The suggested rate is 5–8 litres of air per minute for each cubic metre
of water, coming from at least 2 air stones in different locations in
the fish tank. Densely stocked units may require considerably more. Make
sure that the water is not churned too vigorously or in a way that
disrupts the fish swimming. A clear sign for lack of oxygen is when fish
are gasping for air at the surface. This behaviour, called piping, is
when fish swim close to the surface of the water and take air into their
mouths. This is an emergency situation that needs immediate attention.
Backup (redundant) aeration systems are a valuable asset to an aquaponic
system and can be used during power outages and equipment failures;
simple battery backups for air pumps have saved countless fish
throughout the industry.
Temperature
Fish are cold-blooded and, therefore, their ability to adjust to a large
range of water temperatures is low. A steady temperature within their
correct tolerance range keeps fish in their optimal conditions and aids
fast growth and efficient FCR. In addition, optimal temperatures (and
thus less stress) reduce the risk of diseases. Thermal isolation, water
heaters and coolers help to achieve a steady temperature level, although
these may be costly in areas where energy is expensive. It is often
better to grow fish adapted to local environmental conditions. Each fish
has an optimum temperature range that should be researched by the
farmer. Generally, tropical fish thrive at 22–32 °C while cold-water
fish prefer 10–18 °C. Meanwhile some temperate water fish have wide
ranges, for example, common carp and largemouth bass can tolerate
5–30 °C.
Light & Darkness
The light level in the fish tank should be reduced to prevent algae
growth. However, it should not be completely dark, as fish experience
fear and stress when a completely dark tank is exposed to sudden light
when uncovered. The ideal condition is with indirect natural light
through shading, which would both prevent algal growth and avoid stress
to fish. It is also recommended to handle, harvest or grade fish in
darkness to reduce fish stress to a minimum.