DIY Aquaponics: 5 Best 100% Organic Fertilizers – Fish Safe!

16 chemical elements are known to be important to plant growth and survival. Three non-mineral nutrients — hydrogen (H), oxygen (O), and carbon (C) — are absorbed from the air and water in a process known as photosynthesis (meaning “making things with light”). Plants absorb the other 13 nutrients through their roots. These nutrients and trace minerals consist of:

  1. Nitrogen (N)
  2. Phosphorus (P)
  3. Potassium (K)
  4. Calcium (Ca)
  5. Magnesium (Mg)
  6. Sulfur (S)
  7. Boron (B)
  8. Copper (Cu)
  9. Iron (Fe)
  10. Chloride (Cl)
  11. Manganese (Mn)
  12. Molybdenum (Mo)
  13. Zinc (Zn)

There are not always enough of these minerals in a plant’s environment (e.g. soil, rocks, water, air, etc. ) to ensure maximum growth and crop yields, hence the widespread use of chemical fertilizers by many farmers and gardeners.


If you notice the plants in your aquaponic system are struggling to flower or fruit properly – maybe with shriveled, yellowing, and/or stunted growth, it is likely a sign of nutrient and/or mineral deficiency as detailed in the infograph above. Nutrient deficient, weak plants are more susceptible to disease and pest infestations.

In hydroponics or soil growing, providing these essential mineral elements is relatively straightforward. Just add the proper amount of hydroponic fertilizer based on your system’s water volume. With aquaponics, however, the addition of most synthetic plant fertilizers would result in dead fish. Fortunately, there are safe organic fertilizers that supply these vital nutrients. When addressing nutrient deficiencies in aquaponics — like when combating pests in AP gardens — the bottom-line question is whether the substance is toxic to fish.

Here’s a low down on our favorite organic fertilizers that will green things up and get things blooming in no time.

Our 5 Fav All Natural Non-Toxic Organic Fertilizers

  • 1) Fish Poop (Nitrogen+)

Obviously, fish waste is the primary source of nitrogen and several other nutrients in any aquaponic system. Nitrogen is the first element listed in the makeup of modern-day synthetic fertilizers. Nitrogen is a part of all living cells and is a necessary part of all proteins, enzymes and metabolic processes involved in the synthesis and transfer of energy. It is a part of chlorophyll, the green pigment of the plant that is responsible for photosynthesis.

nitrogen cycle

The nitrogen cycle is the same in soil as it is in water, it is simply a different medium that the bacteria process in. Soil farmers have been using manure to fertilize crops for millennia. Adding more fish to your system will generally help your plants grow more rapidly and produce more seeds, foliage, blooms, and fruits.

Be sure to buy organic fish food or else you’ll probably be adding GMO ingredients to your recirculating system! My favorite kind is AquaOrganic Aquaponics Fish Feed from the folks at The Aquaponic Source.

Bones from shellfish and animal bones will supply calcium and phosphorus to your plants. Like nitrogen, phosphorus is an essential part of the process of photosynthesis. It helps with the transformation of solar energy into chemical energy and is essential for plant maturation, root growth, blooming, and withstanding stress. Calcium, an essential part of plant cell wall structure, provides for normal transport and retention of other elements as well as strength in the plant. This is slow release stuff that must breakdown to become plant-available, so add immediately after starting your system then reapply once or twice a year, depending on plant load. And make sure you purchase steamed and ground bone meal to ensure the quickest release and bioavailability.

bat shit
Amazing bat shit fertilizer – $24 + free shipping

After phosphorus, potassium is absorbed by plants in larger amounts than any other mineral element except nitrogen and, in some cases, calcium. It aids in the building of protein and photosynthesis. Sprinkle some of this 100% natural organic fertilizer made from bat poop or “bat guano” to add both elements to your water garden once or twice a year.

4) Banana Peels (Potassium)

Simply save a few peels from your *organic* bananas, let dry, then pulverize in a coffee grinder or food processor before adding to your grow bed. Boom — free potassium boost. Apply once or twice a year.

Maxicrop is Norwegian Ascophyllum Nodosum (kelp). It is a natural, non-pollutant product made from fresh growing seaweed but in a fully soluble form. It contains very small amounts of the major elements found in chemical fertilizers (nitrogen, phosphorous, potassium) and significant amounts of important trace elements including iron, manganese, zinc, boron, and magnesium. Some say you also need to add chelated iron in addition to Maxicrop due to plant availability reasons, which depend on your pH, water temperature, environmental conditions, etc… tho I have not had any iron deficiencies using only Maxicrop (but see #6 below!)

maxicrop iron
Buy 1 liter Maxicrop plus Iron for $15.95 + free shipping on Amazon Prime

Add the recommended amount (on the label, depending on version you buy) every other month or so. Think of it as a multivitamin for your aqua-garden! You can find this organic supplement, along with other seaweed or sea kelp products at most hydroponic stores and on Amazon. Buying the concentrated powder version, which effectively gives you 65 gallons of this liquid magic, is better value than the pictured bottled product above.

*BONUS* 6) Balanced pH

ph soil meter
Soil PH Meter $15 on Amazon

Okay, this is not really a fertilizer. But maintaining optimal pH (a measure of the acidity or alkalinity of the water) is one of the most important metrics that affect the availability of nutrients to your plants. The ideal range of pH for the nitrogen process is slightly alkaline (7.0 to 9.0), whereas the ideal pH for micronutrient availability is slightly acidic (5.5 to 6.5). The lower pH can impact the efficiency of detoxifying ammonia to nitrite to nitrate. Stabilization of pH between 6.8 to 7.0 is paramount in managing the health of plants and fish simultaneously.

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Urban American Gothic Farmers
Urban American Gothic Farmers by Brandon Martella

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