Fish are where the whole cycle begins. Feeding your fish is the primary way of introducing nitrogen into your system, which is a fundamental nutrient cycle in plant life and the secret to aquaponics’s amazing growth. Nitrogen, in the form of nitrates (NO3-) is a fundamental nutrient required for leafy green, vegetative growth. However, fish don’t produce nitrates themselves. When fish eat, they excrete waste, both diffused through their gills and from their excrements, in the form of ammonia (NH3) which mixes into the water (H20). Normally, this nutrient is toxic to fish (and most other living things). We’ll see exactly how aquaponics deals with this poisonous nutrient in the next section.

Primary Inputs

Fish, like most living creatures, require respiration, hydration, nutrition, safe shelter, and physical exercise. As their steward, your fish need your help to maintain air to breath, clean water to swim in, high-quality food to eat, and a comfortable, roomy tank. Every fish has their own unique needs for all of these parameters. It is important to take this into account when choosing your fish.

  • Respiration
  • Hydration
  • Nutrition
  • Shelter
  • Temperature
  • Space

Fish breathe too! Using their gills, fish pull oxygen out of the water just like our lungs draw oxygen out of the air. It is important to maintain adequate oxygen levels in your water. This is known as Dissolved Oxygen (DO). Gills are the fluid exchange system of a fish. Aside from oxygen, gills also exchange ammonia, carbons dioxide, acids, ions, and water through their gills.

Fish need high quality water to survive. That means balancing pH, hardness (mineral levels), brackishness (salt levels) and other important factors. Every fish has different needs across all of these parameters. Research your target species thoroughly to ensure you can create their perfect habitat.

Fish food is the primary entry point for nutrients into your aquaponics system, so it is vitally important that you choose your food wisely. Fish need a balance of proteins, fats, and carbohydrates, just like we do. Different fish feeds are formulated to meet the needs of specific species. The best fish feeds for aquaponics also have a wide spectrum of macro and micronutrients needed by plants. This is covered in more detail in the section on Software: Plants

Many species of fish prefer to stay out of sight. Without anywhere to hide, they will remain in a consistently stressed state. Small and medium size systems can use living stone, driftwood, and other items to create a unique and beautiful shelter for your fish. Commercial growers cannot provide the fish with hiding points in the tank without creating inefficiencies. Instead, they use shade cloth over the tanks and opaque tank walls to keep the fish from feeling too exposed.

Fish are highly sensitive to temperature, some more than others. For example, koi can survive in a wide range of temperatures from 34-90 ºF, but benefit from being kept between 59-77 ℉. Other fish have very small temperature ranges that reflect their natural habitat. Trout require low temperatures around 55 ºF to grow their fastest. They stop growing entirely above 73 ℉! They are cold-blooded, so they take on the temperature of their surroundings. Temperature also determines the amount of dissolved gases in the water. The cooler the water the more soluble the gas. Remember to keep your fish’s prefered temperature in mind when picking your plants!

Fish require adequate space to grow and thrive. If they are too cramped, they will grow slowly and never reach their greatest potential, not to mention they will be miserable! Don’t make your fish miserable, your plants will regret it. Different numbers are tossed around to determine how to size your tank to your fish. They fall into two broad categories, low-density and high density systems. These categories refer to how tightly packed the fish are in the tank. A simple method is to provide at least 10 gallons per pound of fish for a light density system up to one gallon per pound of fish for a high density system. We recommend following the low density formula for beginning farmers. 

Primary Outputs

Fish eat food and make solid, liquid, and gaseous waste. They breath in oxygen and excrete carbon dioxide and ammonia gases. Unlike land animals that use kidneys to eliminate nitrogen waste, fish rely heavily on their gills to excrete this gas. This waste kickstarts the nitrogen cycle by adding ammonia (NH3) to the water (H2O) in the tank. Solid fish waste comes in two primary forms in aquaculture, suspended solids and dissolved solids. 

Suspended solids are anything from solid fish waste to microscopic clumps of nutrients up to about 2 microns in size. The suspended solid waste is typically collected and removed from the system so it does not rot and create an anaerobic environment, which means an area where non-oxygen breathing bacteria live. More on this in the next section. The waste can also become an irritant for the fish gills! 

Dissolved solids are the ionic minerals, both cation (positively charged) and anion (negatively charged), that are indistinguishable molecules within the water molecules. Fish waste decomposes over time via carbon utilizing heterotrophic bacteria, freeing up the remaining nutrients and increasing the Total Dissolved Solids (TDS). TDS is important to the health of your plants, containing ions like calcium, phosphate, nitrates, magnesium, etc., which are the macro and micro nutrients responsible for the growth and health of your crops. 

After a lifetime of unwittingly assisting you in growing your plants, your fish will give you one last gift in the form of fish biomass (that’s the technical term). They grow and eventually become big fish ready for the dinner table or koi pond.

What are you raising your fish for?

When choosing which fish to grow, you should consider what you are growing them for. These living things are more than just friendly fertilizer factories. The two categories of uses for fish are edible and ornamental fish. 

Edible fish are a great source of healthy protein. Aquaponics is a promising method to increase our ability to raise healthy fish protein in cities and communities where fresh fish is traditionally very expensive. A large aquaculture area is needed in order to properly grow edible fish.

Tilapia are by far the most popular commercially grown edible fish. Tilapia grow fast (12 months from fingerling to harvest), create a lot of waste for the plants and can tolerate harsh water conditions. They love warm temperatures so aren’t as common in colders environments. Many states have laws regarding tilapia as they are considered an invasive species.

Barramundi are very popular in Australia, however we have little personal experience with them. 

Bass are often used in aquaponics, but they are more temperamental and grow slower than tilapia.

Like the bass, they are temperamental. This is a cold weather fish that needs pristine water quality. It is only recommended for expert growers during the coldest months. 

Ornamental fish are heavily dependant on your water pH. However, there are some very hardy varieties that can bridge a wide span of pH range. There is also a distinction between aquarium ornamentals and large ornamental fish like koi.

Can Adjust to any pH
Super hardy, beautiful, and friendly, with great personalities.
Xiphophorus (Platys)
Cheap and hardy. This is what we started with back in Boston.
Semi aggressive, but usually fine with other non-aggressive fish. Very beautiful.
Everyone is like, “meh guppies,” but there are some really pretty tailed guppies out there. Nothing prettier than schooling guppies, in our opinion.
Generally any livebearer (Endler's livebearer for instance) will be good for high pH

Neutral-low pH gives you the biggest range of tropical fish
Great community fish and the right varieties are very hearty. We enjoy Candy Cane and Purple Emperor Tetras.
Freshwater angelfish
Come on, angelfish are beautiful and are only semi-aggressive! Easy care level.
Zebra Danio
These are pretty, can adjust to pH well, and affordable to boot!
Beautiful and hardy, but VERY aggressive. We would only put this in with schooling fish (tetras, not guppies) and catfish. No fancy finned fish! They also don’t like fast moving water, which tears their delicate fins.

The Cleanup Crew
For cleaning up algae, diatoms, and other microscopic critters that compete for your plant’s precious nutrients.
Amano shrimp
These guys are the best algae cleaners we've ever seen. They are hardy and just plain cool. Great addition to an aquarium tank.
Otocinclus Catfish (Oto)
This is the WORKHORSE. Another great scavenger. These guys will go to work on any algae or diatom problem. They like to be in groups. They are sensitive though, so take time to acclimate them. See acclimation SOP.
Pygmy Corydoras Catfish
So tiny. So cute. They are social fish and like to be in groups of 4 or more. They will eat any fish food that the fish don’t take care of.
Bristlenose or Clown Pleco
These guys are pretty shy and we only recommend one to a small aquarium, but they are cool and will chill out on some driftwood. They tend to take the night shift, doing most of their scavenging after the lights go out.

Note for Commercial Growers: Multiple Cohorts

If you are contemplating growing fish for commercial purposes, ensure your facility is large enough to house multiple cohorts of fish. Multiple cohorts, or growing fish in different age groups, allows you to harvest a crop of fish without totally disrupting your nutrient load in the system. An example would be Joe, who has a total production capacity of 1000 koi, which he sells as high-end ornamental fish. Joe keeps four separate tanks of 250 koi each. Today, the first tank houses fingerlings (baby fish), the second tank houses juvenile fish, the third tank houses adolescent fish, and the fourth tank holds the adult fish, ready for harvest. At the next harvest three months from today, Joe will repopulate the fourth tank with new fingerlings because the fingerlings in the first tank are now juvenile stage. Of course, the specifics are dictated by your goals and needs.

Here is a great resource by LaDon Swann, Ph.D. about fish requirements from a fish farmer’s perspective.

Read on about the Aquaculture Hardware here.

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