Starting an aquaponic system

This experiment is an introduction into the functions and management of a small aquaponic system. Like a home aquarium, an aquaponic system requires special attention in the first weeks, since the microbial community in the water and in the plant boxes need some time to get established.

Learning goals

  • Know why an aquarium requires a filter.
  • Know how to monitor the water quality by using simple aquarium test kits.
  • Be able to explain what the measured factors indicate in terms of water quality, fish health and plant growth.
  • Understand how to react in case of bad water quality in the aquarium.

Background information

There are at least three species of living organisms in an aquaponic system. There are fish, plants and bacteria. In our small classroom system we will have about four goldfish in the aquarium, 60 plants in the plant beds and about 100'000 billion of bacteria and other very small species in the gravel or LECA. Bacteria have been on this planet for 3 billion years while man exists for at most 3 million years, and our civilisation for just 10'000 years. A human being couldn't survive a day without the help of bacteria and neither would the fish and plants in our classroom system.

The fish need oxygen to survive - in the same way as human beings, i.e. by breathing - but the fish take oxygen from the water and excrete ammonium and carbon dioxide over the gills. There is also ammonium in the excrements (faeces) from the fish. The ammonium in the water can become dangerous to the fish. The bacteria can transform ammonium into a substance (nitrate) which is harmless to the fish and at the same time an important plant nutrient. This process is called nitrification and it is necessary to supply the plants with nitrogen fertilizer.

So the first thing we need to do before we put any fish in the aquarium, is to start growing a lot of bacteria in the plant boxes, so that they can take care of the fish excrements i.e. the ammonium in the water.

Another important factor influencing the water quality is pH. It indicates whether the water is acidic, neutral or basic. The pH is measured on a scale of 1-14 with 7 being neutral. Is the pH lower than 7, the water is acidic, if it is higher, the water is basic. Depending on the fish species, the optimum pH varies. Goldfish tolerate variations in pH much more than other fish species, but to avoid stress, the pH should stay within the range of 6.5 - 8.

Time requirement

The starting phase should last at least for three weeks.

Material requirement

Aquarium test kit for nitrite and nitrate
  • 1 classroom aquaponic system (see experiment 1 in this teaching unit)
  • 1 aquarium test kit for ammonium (NH4+)
  • 1 aquarium test kit for nitrite (NO2-)
  • 1 aquarium test kit for nitrate (NO3-)
  • 1 aquarium test kit for pH
  • 1 bacteria starter package for aquarium filters
  • 1 aquarium thermometer

Let's start

  • Start the biofilter - multiply the bacteria! Fill the system with water and start the air pump. It is important to keep the oxygen level high for the bacteria to thrive. The water pump is now circulating the water through the system. There are different ways to multiply the bacteria. For example you can add a starter package of bacteria, or you add a very small amount of ammonium to the system in order to feed the already present bacteria in the substrate. We suggest to add a fertilizer containing ammonium, to achieve an ammonia concentration of 10 mg/l. If using ammonium sulfate, you would add 4.7 g of the powder to 100 l of aquarium water.
  • Is the filter working? If there are enough bacteria in your filter to transform ammonium into nitrate, your filter is working. To find out, you need to measure ammonium, nitrite and nitrate. You can begin with measuring the ammonia concentration and monitor, how every day it is decreasing. After a week you can start measuring nitrite and nitrate, using the sticks from the pharmacist or the aquarium dealer. During the first days you can have some indication of nitrite but when the system is ready you should have no response of nitrite. It normally takes between three to six weeks to multiply the bacteria enough so they can digest the ammonium amount that will be caused by the application of fish food.
  • Control and regulate pH! To measure the pH in the water use the pH-sticks. If the value is between 6.5 and 8, there is no need to do anything. If it is above or below that value, change part of the water! If you feel experienced enough, instead of changing the water, you could add formic acid in case the pH is higher than 8. Add one part of the acid to 50 parts of water and use a drop of this solution every day until pH is right. If pH is too low, you can add a tea spoon of calcium hydroxide or potassium hydroxide in a cup of water and use a drop of this until pH has the right level.
  • Temperature: Temperature is very critical because it influences other water quality variables. For example cold water can hold more dissolved oxygen than warm water. Fish are very sensitive to changes of temperature, smaller fish being more sensitive than larger ones. Goldfish belong to the group of coldwater fish and prefer temperatures between 18 -25 °C. They will tolerate any temperature from 4-30 ºC however, as long as the water still contains enough oxygen and low amounts of ammonia. Remember that the body temperature of fish is always the same as the water temperature. Therefore, in colder water fish will eat less or stop eating at all when the temperature drops below 9 °C (Mette, 2006). On the contrary, in warmer water fish will digest faster and get a lot more hungry. Be careful, though, not to give too much feed! More feed means that the fish need more oxygen, but in warmer water there is less oxygen. So these two factors are working against each other, and you have the balance in your hand.

See and feel

  • Ammonia (tolerance value: < 0.8 mg/l) and Nitrite (should be 0 mg/l at all times, max. 0.2 mg/l) (measure each second day).
  • Nitrate - its raising indicates that the filter process has been started (once per week). The nitrate value should be between 10-100 mg/l.
  • pH should be between 6.5 and 8 (once per week).
  • Water temperature should be between 18 - 25°C, depending on the fish species
  • If the values of ammonia, nitrite, nitrate and pH are out of range, stop feeding. In severe cases, change part of the aquarium water.

Didactical comments

In order to understand the process of nitrification, or what happens in the filter, some basic knowledge in chemistry is required. However it is possible to explain it in using metaphors: two little animals (bacteria) are responsible to transform the poison (ammonium) into a non toxic substance (nitrate), which at the same time will feed the plants. With the ammonia and nitrite test kit we indirectly measure the existence and quantity of these bacteria.