In an aquaponics setup, your plants get most of their nourishment from the waste your fish produce, while the plants in turn clean that water before it flows back to the fish tank. It’s a partnership that benefits both sides, but it isn’t automatically perfect. To keep everything running smoothly, you need to check your water regularly and top up any nutrients that run low.

Testing Your System Regularly

Whether your aquaponics setup is brand new or has been running for years, you’ll want to check the water for several different chemical markers. Almost any basic testing kit will work, as long as it measures pH, water temperature, and electrical conductivity (EC). Getting any of these three out of balance can create real problems.

Temperature Swings

When the water gets too hot or too cold, your fish become stressed and tend to eat less. That makes them more vulnerable to illness and, in extreme cases, death. Poor temperatures can also harm root health and slow down plant growth. If things get bad enough, the beneficial bacteria that turn ammonia into nitrates can be wiped out, which threatens the whole system.

Off-Target pH

An incorrect pH also stresses your fish and raises their risk of disease. On the plant side, an off pH makes it harder for roots to take up nutrients, which slows growth — and just like with temperature problems, it can knock out the bacteria responsible for converting ammonia into nitrates.

Electrical Conductivity Issues

When EC drifts too far in either direction, plants may grow unusually slowly or, in some cases, unusually fast. Either way, the result tends to be a lower-quality harvest and a greater chance of your crops bolting prematurely.

Spotting Common Nutrient Deficiencies

It helps to know in advance which nutrients your system is most likely to run short on. Keep in mind that the species of fish you raise and the food you feed them both influence which elements end up missing, since your plants’ nutrition ultimately comes from what the fish consume and excrete. The core elements your plants rely on are nitrogen, phosphorus, potassium, calcium, sulfur, and magnesium, along with smaller trace amounts of boron, copper, chloride, zinc, molybdenum, iron, and manganese. Some of the deficiencies you’re most likely to encounter include:

• Iron — Watch new growth closely. If fresh leaves come in pale or yellowish, your system’s iron level is probably too low.
• Potassium — Plants take up this element in soluble form and use it to drive many of their internal processes, including moving water through the plant and controlling how gases move in and out of leaf tissue.
• Calcium — This mineral plays a structural role in plant growth; without enough of it, plant tissue can’t hold water properly and may start to break down. Calcium uptake can also be thrown off by too much magnesium or potassium in the water. In humid, poorly ventilated conditions, plants can show calcium deficiency symptoms even when calcium levels in the water are adequate — you’ll notice they aren’t losing much water and aren’t taking much up either.
• Phosphorus — Without enough phosphorus, expect stunted growth, especially early in a plant’s development. Leaves may darken or take on a reddish or purplish tint. Phosphorus shortages can also stop plants from flowering or producing fruit and vegetables altogether.

How to Correct Nutrient Shortfalls

The right fix depends on which element your system is short on.

Potassium

There are two main routes for adding potassium back into your system:

• Foliar spray — Mix up potassium chloride and spray it directly on the plants. Plan on repeating this at least once a week.
• Feed additive — Work potassium into the fish food itself using a kelp meal concentrate, or use potassium sulfate or potassium hydroxide instead.

One catch with potassium: what you add isn’t always usable by your plants. Potassium competes with calcium and magnesium, and an excess of either can reduce how much potassium your plants can actually absorb. To get the most out of any potassium you add, keep an eye on your calcium and magnesium readings and make sure they stay balanced.

Calcium

A spray is generally the most reliable way to correct a calcium shortage. Dissolve calcium chloride in water — a good starting point is about 4 teaspoons per gallon — and spray your plants once a week, adjusting the dose upward if needed. Adding crushed shellfish shells to the fish tank is another option, since it boosts both calcium and phosphorus over time.

Iron

Iron needs to be supplied in a form plants can actually absorb, which means using a chelated iron product — Fe-DTPA is a solid choice. It works best when your pH sits at 7.5 or below, which is the range you should be maintaining anyway. Target roughly 2 mg per liter, and calculate your total tank volume so you can dose the right amount roughly every 3 to 4 weeks.

Phosphorus

Phosphorus is another essential building block for healthy growth, and like iron, it needs to be in a plant-available form. Rock phosphate is one of the easiest options and is usually available at a local garden center. You can apply it straight to the grow beds, letting roots absorb it directly. Keep the treated area shaded from direct sun so the phosphate doesn’t break down before your plants can use it. Aim for a range of roughly 20 to 40 ppm per plant.

Key Takeaways

The encouraging news is that most aquaponics systems run well with minimal intervention once they’re established. Iron deficiency tends to be the most frequent issue you’ll run into. As long as you keep an eye on your plants visually and back that up with regular testing, you’ll be able to add the nutrients above as needed and keep your system healthy and productive. Above everything else, pay close attention to your pH — it has the biggest influence on how well your plants can take up the nutrients you provide. Note that adding supplemental nutrients like these is typically only necessary for larger, commercial-scale aquaponics operations rather than small home setups.