Plant Nutrient Management in an Aquaponics System

By: Tawnya Sawyer

(This is a reprint of an article I wrote that ran in

When I first stumbled upon aquaponics in early 2009 it was while I was still running product development, and by extension the plant laboratory, for AeroGrow International. While I am not a scientist by education (my undergraduate degree is in Agricultural Economics) I learned a tremendous amount about hydroponic nutrients in my various plant research roles red pepperat AeroGrow. And one thing was for sure. There was absolutely no way that fish waste could be the sole basis for a complete hydroponic plant nutrient. I’ve obviously changed my tune since that day. Through my community site at and my own experience, I have learned that not only can you grow plants with nothing but fish waste, but that a mature aquaponics system grows many plants better than can be grown hydroponically. The key to this amazing result lies in three important factors: a healthy bio-filter, pH within proper range, and a high-quality fish feed.

The role of the bio-filter in a hydroponic garden: you add mineralized nutrients that are immediately available to your plants. In aquaponics, the nutrients must be converted to a form that the plants can uptake. This conversion is done by nitrifying and heterotrophic bacteria, and ideally composting red worms, all residing in your media bed. These bacteria and worm colonies, and the media they call home, make up what is commonly referred to as the ‘bio-filter’. Think of the bio-filter as the carburetor in your car that takes gas and makes it an available energy source to power your engine. Establishing the bio-filter begins when ammonia is first added to the system, either by fish or synthetically, in a process referred to as ‘cycling’. Cycling is complete when enough bacteria have established themselves to convert all the ammonia, and all the subsequent nitrites to nitrates and the levels of ammonia and nitrites are zero. But even after cycling is complete and you have a functioning bio-filter, your plants still may not be getting enough nutrients. This is because the bio-filter matures over time, and becomes increasingly more efficient at converting fish waste into plant food.

In the first three months of an aquaponic system’s life, you may only be able to grow plants well that require a low level of nutrients, such as salad greens. However, by the time your bio-filter is six months old, your system ‘carburetor’ will be running at full tilt and be making a full range of nutrients available to your plants. At this point you should be able to grow just about anything, including heavy feeders like tomatoes and cucumbers.

The Key Role of pH

Just like with a hydroponic system, or any growing system for that matter, pH plays a key role in a plant’s ability to take up nutrients. If pH gets too far out of the range where a particular element is most available to the plant, the plant will become deficient in that element. Most of the nutrient deficiencies I see in aquaponics are not deficiencies at all. They are pH problems. The deficient element is in solution, the plants are just not taking it up.

Please take a look at the chart above. The horizontal axis is pH. Each horizontal line above the x-axis shows the relative ease plants experience taking up the element named, based on the pH of the water solution in your system – the thicker the line the easier the uptake. Notice that the sweet spot on the chart is is between pH 6.0 and 7.0. This is the spot where every line has a thicker section. This means that if your pH stays in this range, your plants will have the best chance to take up all the nutrients they need.

In aquaponics, pH actually becomes a compromise between the optimal pH for the plants, the fish and the bacteria. Since fish and bacteria prefer a pH more in the 7.0 – 8.0 range, aquaponic gardeners typically target a pH of 6.8 – 7.0 in their system. This seems to work out pretty well for everyone.

When you adjust pH in an aquaponics system, whatever you do – Do It Slowly! Fast, large pH swings are very stressful on fish and will be much more of a problem than having pH that is out of range. Target shifting your pH reading no more than 0.2 per day and you should be fine.

The safest way to lower pH is to use a product called ‘pH Down’, which is likely sold at your local hydroponics store. Any type is fine, except avoid anything with citric acid as it is anti-bacterial! (We carry our own version of ‘pH Down’ called AquaDown which is aquaponics-safe and feeds your plants phosphorous as you lower your pH.)

The best way to raise pH is to alternate calcium hydroxide – also known as ‘hydrated lime’ or ‘builder’s lime’ – with potassium carbonate (or bicarbonate) or potassium hydroxide (‘pearl ash’ or ‘potash’). This has the added benefit of also adding calcium and potassium to your aquaponics system, something your plants will appreciate. (We carry our own ‘pH Up’ kit called AquaUp containing both calcium and potassium carbonate.)

You can also use crushed egg shells, snail shells, or sea shells to increase pH more naturally, but be sure to boil or bleach them to prevent getting any unwanted bacteria into your system. Also be sure to contain them in a removable bag (nylon hosiery works well for this) so you can easily remove them if the ph goes too low. These materials affect the pH slowly as they decompose, but since this decomposition usually doesn’t happen evenly they can also cause sudden pH spikes.

Use a High Quality Fish Feed

I recommend finding the highest quality fish feed you can afford that has been formulated specifically for the kind of fish you are growing. Not only will this go a long way to insuring the health of your fish, but since the plants are being fed from the waste of the fish, it will go an equally long way to insuring that your plants have everything they need.

Do I need to supplement?

You should not need to supplement, or supplement only rarely, if you follow my recommendations in this article, my two prior articles in Urban Garden and on my website for building a flood and drain style aquaponics system with a 12″ deep grow bed. In fact, adding supplements might be harmful to your fish. Because aquaponics is a natural ecosystem, the best thing an aquaponic gardener can do is get the system well set up; manage pH, oxygen, and water temperature; feed the fish and then leave it alone!

The only time that you might see some nutrient deficiencies is in those first six months when you are establishing your bio-filter. Aquaponic gardeners typically use two products to help them through this period. The first is a liquid seaweed product called Maxicrop. This safely adds a number of micronutrients to your aquaponics system to help your plants become established before your bio-filter is fully turned on.

The second product is chelated iron. Because iron uptake is especially sensitive to pH, iron deficiencies sometimes appear in an aquaponics system that insists on staying at a high pH (see the chart above). Chelated iron is generally considered harmless to the fish and is a useful boost if you are starting to see yellowing on your plant leaves. (We carry a version of chelated iron that works effectively within the pH ranges of aquaponics systems called AquaIron.)

Other than that, enjoy the fact that you will have no issues with salt and nutrient buildup in your aquaponics system, and that you will never have to dump and replace those nutrients. Because an aquaponics system is first and foremost an eco-system, consider yourself not the dictator of your system, but instead as the conductor of a natural process.