Aquaponics – Guarding the Plant Growth Medium, by J.F.


The funny thing about growing fish in your backyard (Aquaponics) is everyone thinks about the fish and the right water– temperature, pH, chemical balance, replacement, and so forth– for the fish. No one thinks about the plants and asks questions, such as:

* What is the right environment for the plant root system?

* What should one do about moving the plant growth medium and pulling up dead plants?

* Should water be added to the plant growth medium or should it be added to the fishtank when water needs to be replaced?

This lesson addresses both how to correctly add water to the system because of evaporative losses, and how to keep the right temperature of the plant growth medium for the plants (not just the for the fish).

Having a handle on these concepts ensures that one can spend more time enjoying the fish as they play in the water, observing the spawning process, and harvesting the fish and vegetables and less time with cultivation, emergencies, and disease.

The following two principles are keys that make Aquaponics easier than it already is. One should remember that farming fish and plants within an enclosed system is a fairly simple way of providing an organic and sustainable food source that can feed a family for years.

Adding Water to the System

Water in the indoor system evaporates more slowly than the outdoor fish tank, which naturally encourages evaporation of water from the system. In contrast, the indoor fish tank operates with a slower rate of evaporation. In a typical Aquaponics set up, using the industrial bulk container (IBC), the fish tank water level runs at about 170 gallons. Over a week’s time, the outdoor system will lose approximately 10 to 15 gallons of water (depending upon the outside temperature, the placement of the return water hose, and the duration of the sunshine on plant growth medium).

No matter the rate of water loss for the outdoor or the indoor system, the water must be changed at a minimum of 10 to 15 gallons or at least between 6% and 9% of the total fish tank capacity per week. Most times meeting this requirement means removing water beyond what has already evaporated. Use a 5-gallon tote bucket or a siphon, while watching the water level on the fish tank decrease until the level shows between 155 and 160 gallons for the standard IBC. For the larger tanks, drain the water until it is down between the 6% to 9% of the total fish tank capacity. (Do not include the plant tank capacity in this situation.)

The drained water can be used for an earth garden, to replace toilet water, or where other gray water can be used. In all cases, the water can be used for drinking, if first purified using normal purification processes. (If it cannot, then the fish and plants are suspect also.)

Replacing the water is fairly simple. Fill up the 5-gallon bucket with fresh water from a source you trust and add the de-chlorine and de-chloramines liquid, following the instructions on the label. Then return the water to the fish tank, NOT the plant tank containing the plant growth medium.

This action is key and worth repeating: Return the water to the fish tank, NOT the plant tank containing the plant growth medium.

Pour water at a slow rate or a fast rate into the plant growth medium and you move the plant growth medium. Move the plant growth medium and nothing happens to the plant growth medium. Move the plant growth medium when plant roots are surrounding it and you fundamentally alter the root structure of the plant causing it stress and usually the loss of the fruit and possibly the plant as well.

Some may suggest that pouring water straight into the fish tank has worse consequences to the fish than it does to the plants because the fish will be more disturbed than the roots. They will not. They live in water and the addition of water to the tank may stir up debris on the bottom of the tank, so that for a few hours the tank water may appear murky. No harm comes to the fish. On other hand, adding water to the plant growth medium where roots are disturbed and moved from their position does harm the plant. It can cause the loss of fruit, leaves, and even the plant itself. Remember the roots of the plants have grown to an area that is most advantageous to the plants.

In an emergency, water can be added to the plant tank at a very slow rate in an area that appears to have no plant growth and no root attachment. Never add fast flowing water to the plant growth medium.

In summary, add water to the fish tank, NOT the plant tank.

Keeping the Right Temperature of the Plant Growth Medium

The average minimum temperature of the plant growth medium for vegetable seeds to germinate is 40 degrees Fahrenheit (F). There are some higher minimums (like 60 degrees F). The average optimum temperature range of the plant growth medium for vegetables is 45-80 degrees F. Plant growth medium at a temperature above 95 degrees F can cause the plants to start shedding their fruit and dying (to say nothing of the fish stress at this temperature).

A plant tank that sits upon the indoor fish tank presents little challenge for maintaining optimum plant growth medium temperatures. Plant growth medium in an indoor plant tank maintains a relatively stable temperature equal to the temperature of the fish tank water, which is anywhere between 65 degrees and 80 degrees. A grow light situated above the plant tank has a minimal heating effect upon plant growth medium.

Outdoor plant tanks present a challenge when in direct sunlight. The sun heats up the top of the plant growth medium to temperatures above 100 degrees F, causing stress to the root system of the plants, which means dropped fruit and eventually a dead plant.

There are two main possibilities to prevent overheating of the plant growth medium of an outdoor plant tank: 1) build a shade around the plant tank that limits the amount of direct sun light on the medium and also on the plants, or 2) plant ground cover vegetables on all of the exposed plant growth medium.

The first method of preventing overheating is doable. One builds a framework around the plant tank made of light wood. This frame is then surrounded by a fabric that allows the wind to blow through but moderates the rays of the sun. This adds to the cost and maintenance of the Aquaponics system. Also, heavy winds can and do destroy these shades.

The second method for preventing overheated plant growth medium is easier, far less costly, and has minimal impact. Scatter lettuce and kale seeds throughout the planting medium before planting any other seeds. After 3 weeks the lettuce and kale seeds have sprouted. The stems are one to three inches high. The leaves are out and cover the planting medium. This means the planting medium is shaded from the sun, which means less evaporation. Less evaporation means greater control of water resources. Shade means less heat is transferred to the medium and more to the plant itself.

At this point, one can plant whatever other vegetable seeds are desired– brussel sprouts, onion, garlic, carrots, melon, pumpkin, egg plant, tomatoes, et cetera. The roots will not be heated beyond optimum growing temperature. In addition, if the evenings are cool to cold, the lettuce and kale provide an insulation layer between the cold night air and the warmer water. The insulation prevents excessive evaporation, in turn, reducing the cooling affect, keeping the roots within the optimum temperature range.

Introducing non-living organic material (hay, newspaper, cardboard) to insulate the plant growth medium adds excessive waste to the enclosed system and can harm the fish environment. Introducing plastic sheeting across the top of the plant growth medium creates a solar effect, expanding the sun’s heat signature across the plant tank raising the plant growth medium’s temperature beyond optimum levels, introducing additional algae growth, causing competition for nutrients and reducing the grow area. The only time plastic sheeting laid across the growth medium should be considered is when the plant tank is outside in normally colder climates, but that is a lesson for another time.

The key to effortless management of plant growth medium temperature optimization for root success is to create a natural layer of insulation using edible plant coverings, in this case, lettuce and kale.


Managing an Aquaponics system should be effortless. This means enjoyment of watching the fish play and having the fruit and vegetables grow with minimal labor. Guarding the plant growth medium protects the roots of the plants. Protecting the roots means stress-free plants, which means harvest abundance.

One must only remember two principles when it comes to maintaining a healthy growth medium. The first principle is how to correctly add water to the system because of evaporative losses; that is adding water to the fish tank, NOT to the plant tank. The second principle is how to keep the right temperature of the plant growth medium for the plants’ success (not just the for the fish); this is accomplished by covering the plant growth medium with an insulating layer of a living vegetables, such as lettuce or kale or both. Doing so keeps the plant growth medium within the optimum temperature range during the day, when the sun can heat the growth medium beyond 100 degrees F and can drop below 40 degrees F during the night time.

Following these two principles will keep your Aquaponics system producing healthy and hardy vegetables.