Fish Health & Care

In general fish require the following to flourish: good water quality (includes filtration, and high dissolved oxygen (DO)), consistent temperature and a quality food source. More specific conditions are dependent on each individual species. These conditions must be balanced with the needs of the other two organisms within the aquaponics system (plants and bacteria). Fish health in general can be measured by their ability to feed. If fish are not feeding then there is evidently an issue. We will now look at the main factors that will contribute to healthy fish.



Each species of fish will have an ideal temperature range and an optimum temperature within this. This optimum temperature may not always be met as it can conflict with the temperature range of the plants or the bacteria. However, a compromise can be reached, in general, at 23° C.  Outdoor systems will usually however be lower than this.

Any major changes in temperature can have adverse effects on fish health, particularly when reaching extremes of temperatures during the day and night cycle. It is important to know the optimum range of the fish you are growing, as some fish will stop feeding at a certain temperature, and some will suffer from thermal stress and may die. For example, some Tilapia will stop feeding at 18° C and can perish below 12° C.

When adding water or topping up (replenishing the water lost through evaporation), it is important to do this slowly to afford the fish a chance to adjust to the temperature change. When adding the fish to the water initially, it is important to let the fish acclimatize to the new conditions and temperature.

Be aware of the year-round temperature in the local area and select a fish that will thrive in all seasons. It can be expensive to use cooling or heating equipment, and this may be unnecessary if the fish are selected properly. Using insulation and covering the reservoir (fish tank) will help to stabilize the temperature. Be aware that higher the temperature, the lower the in the water: this will have a negative impact on the fish, bacteria and plants.

The pH range for fish is higher then the range of pH for plants but close to that of the bacteria. The balanced pH range in an aquaponics system is 6.5.0 – 7.5.



The ideal pH will also depend on the specific species being used in the aquaponics system. Any extremes of pH or major swings (more than 0.3 over a few hours) will have a negative impact on fish health, and can cause stress or even death. If the fish are stressed they will be more susceptible to disease or death. Most freshwater fish will do well in a pH range of 6.0 – 8.5. Fish will adjust to different pH ranges, but only if done very slowly.

In aquaponics the plants will produce some oxygen through photosynthesis during the day but they will produce carbon dioxide at night. The carbon dioxide will lower the pH hence pH may be lower in the morning than the afternoon.


Ammonia, Nitrite and Nitrate

As discussed previously, ammonia, nitrite and nitrate are fundamental compounds of the Nitrogen cycle and therefore fundamental to aquaponics. Of the components, the fish are the most sensitive organism to the levels of these compounds, thus fish dictate the ‘safe’ levels within the aquaponics system.

Ammonia is toxic to fish; anything over 0.25 ppm (parts per million) can be harmful and cause stress, which can leave the fish more susceptible to disease. Nitrate will also be harmful at 0.5 ppm or higher. Nitrates should be kept at 20 ppm or lower although on a short-term basis they can be higher.

Ammonia poisoning is known to cause extensive tissue damage, in particular the gills and kidney, physiological imbalances, decreased resistance to disease, impaired growth and even death. It can be seen physically by reddening of the eyes.

Nitrite poisoning inhibits the uptake of oxygen by red blood cells also known as brown blood disease.

Fish Feed

Fish feed is one of the main inputs within an aquaponics system. The content of the fish feed is the main source of protein and nutrients for the fish and eventually the nutrients for the plants. We recommend using a commercial organic feed with a high protein source or an organic alternative feed source.

One of the most common problems in any aquaponics system is over feeding. This is when the fish cannot eat the amount of feed given to them. This will result in cloudy water, excess ammonia and fish health problems. We recommend simply feeding them a set amount, and after 30 minutes if food is left uneaten, remove it and reduce the feed amount the next day. It may be more beneficial to feed smaller amounts twice daily. If the ammonia levels are rising or ‘spiking’ it will be necessary to stop feeding for a day or two in order to allow the system to stabilize. Then simply resume feeding at a lower rate.

Fish feed will depend on the focus of the aquaponics system, whether that is plants or fish. We recommend that in starting off in aquaponics, the focus should be on the vegetable production rather then fish. Higher filtration and more space is needed for intensively growing fish. Fish feed charts are available online for the specific fish being grown. Regular fish pellets are ideal for fish feed although alternate feed sources will be discussed in a later article.

Feed conversion ratio (FCR) refers to the amount of food eaten by the fish relative to the overall poundage of meat produced. Protein is the most important aspect of fish feed. It provides energy and supplies amino acids (Amino acids are the building blocks of protein in the fish’s cells). Protein requirements are generally higher in smaller fish. As fish grow larger, their protein requirements usually decrease. Fish are capable of using a high protein diet, but as much as 65% of the protein may be lost to the environment. We recommend a fish feed with a protein content of 25 – 40% for edible fish, decorative fish will have lower protein requirements.


Fish Selection and Profiles

 We recommend using Gold fish or carp if you are starting off in aquaponics as they are the most robust fish and produce large quantities of ammonia which is ideal for aquaponics. As you become more comfortable with ‘water quality’ and the balancing of fish and plants you should move into edible fish. Below we have listed a few types of fish and their specific parameters.



Latin: Oreochromis spp.

Tilapia is a hardy fish and highly recommended for use in aquaponics. Tilapia is primarily cultured because of its high adaptability. Tilapia has a high growth rate and frequent spawning tendencies. It is thus an ideal fish for aquaponics. However, it is a tropical fish and so it requires a heated tank, which can be expensive.

pH range: 7-8

Temperature range: 12° C – 30° C (under 18° C they will cease eating)

Optimum temperature: 26° C

DO levels: 3- 10 mg/l (can survive as low as 1 but not advised)

Growth time: Under ideal conditions can grow 1.13 Kg in 6 months


Rainbow Trout

Rainbow trout has many characteristics that make it suitable for aquaponics: it is easy to farm, it is quite resistant to diseases. However, this fish can be more sensitive to environmental factors than some of the other factors and so should only be grown by an experienced fish keeper.

pH: 7- 8

Temperature range: 5 -24° C

Optimum temperature: 23

DO levels: not less than 5.5 mg/l,

Growth Time: roughly 12 months



Latin: Silirformes

Most catfish are bottom-feeders, meaning they swim and eat at the bottom of the tank. There are many varieties of catfish and the specific requirements should be investigated further. They are negatively buoyant, which means that they will usually sink rather than float due to a reduced gas bladder and a heavy, bony head. Catfish have high heat tolerances just like tilapia, but they can also thrive in the cold.

pH range: 5.0 – 7.0 (can live in 5.0 -9.0)

Temperature range: 4° C – 35° C

Optimum Temperature: 23° C – 30° C (under 10° C they will cease eating)

DO level: Minimum of 1 mg/l but 4 mg/l is ideal for growth

Growth time: Harvested at 0.5 Kg in just over 6 months in ideal conditions.


Common Carp or Goldfish

Latin name: Cyprinu carpio

Common carp is a popular pond fish. Common carp are used as a pond fish because they are very disease resistant, tolerate wide temperature and pH range, and will accept different food types. They are ideal for starting with.

pH range: 6.0 – 9

Temperature range: 1° C- 32° C

Optimum temperature: 12° C- 24° C

DO level: 4 mg/l and a maximum of 10 mg/l

Growth time: 0.6 Kg in just over 6 months in ideal conditions.


Daily and weekly maintenance

Aquaponics systems should be very low maintenance compared with ground-grown crop production and aquaculture. Feeding and water quality are the main inputs that will take up the user’s time after the initial build. Water quality testing should be done weekly in an established system (once it has fully cycled), and more frequently in a new system is just beginning (being cycled). pH should be monitored every other day as this will be a key early indicator of problems occurring in the system.



Feed (as above). Remove any feed that is uneaten after 30 minutes and reduce feed ration for the next feeding (or alternatively feed the same amount but split it between morning and evening).