Frequently Asked Questions

Aquaponics combines aquaculture (raising fish in recirculating systems) with hydroponics (growing plants in water without soil). Fish waste feeds the plants and the plants help clean the water for the fish, with beneficial bacteria converting the waste into plant-available nitrates along the way.

The FAO notes that aquaponics "can use up to 90 percent less water than traditional irrigation systems" — a figure we cite throughout the site because it comes from an FAO workshop report rather than folklore.

Read our complete beginner's guide →

Hydroponics grows plants in water with added nutrient solutions — fertiliser dosed by the grower.

Aquaponics replaces that fertiliser with fish feed: the fish eat, their waste is converted by bacteria into nitrates, and the plants take up those nitrates as nutrients. New Mexico State University puts the water saving in perspective: traditional aquaculture discharges 10 to 20 percent of its water every day, while an aquaponic system uses about 1% of the water that pond culture needs for equivalent tilapia yields.

Aquaponic systems normally run without pesticide or herbicide sprays, precisely because a spray meant for pests can kill the fish — both NMSU and Cornell describe this pesticide-free way of operating.

Whether that counts as "organic" is a certification question, not a description of the method: labels are set by certification schemes, and we do not claim any certification here. If the label matters to you (for selling produce, for example), check the rules of your local certification programme.

We deliberately do not publish a dollar figure here. The honest answer from NMSU's extension guides is that, depending on the size and sophistication of your system, aquaponics "can require a substantial capital investment", plus ongoing energy inputs and a certain level of skill to manage well. Anyone quoting a flat "start for $200" number is selling something.

Start small: our build guide walks through the components and the real economics, and the planner turns your plant choices into a shopping list you can price locally.

NMSU lists the species well adapted to recirculating systems: tilapia, channel catfish, rainbow trout, perch, common carp, Arctic char, goldfish, and largemouth and striped bass. The most common choice in home and commercial systems is tilapia — a warmwater fish that tolerates crowding and fluctuating water conditions. If you mainly want plants, NMSU points out that non-edible ornamentals such as goldfish work perfectly well as the nutrient source.

Check before you buy: tilapia and other non-native species are regulated in some regions, so confirm what you may legally keep where you live. Temperature is the real deciding factor — warmwater species want 65–85°F water, coldwater species like trout need 55–65°F.

See our complete fish guide →

Start with plants that have low to medium nutrient needs. NMSU names lettuce, basil, spinach, chives, other herbs, and watercress as well adapted to aquaponic systems, and Cornell's home guide grows lettuce and kale as its reference crops.

Fruiting plants — tomatoes, cucumbers, peppers — are the exception: NMSU says they have higher nutrient requirements and "will only do well in aquaponic systems that have high stocking densities of fish", which a new backyard system does not have.

Tip: leafy greens and herbs are the forgiving first crop while your system matures. Our planting guide covers what to plant and how.

Two groups to leave out of a small system:

• Heavy feeders in a young system — tomatoes, cucumbers and peppers need high fish stocking densities (see above), so they disappoint beginners.
• Plants that need acidic water — the nitrifying bacteria that make the system work "basically stop working when pH falls below 6", so an aquaponic system has to sit near neutral. Crops that demand strongly acidic conditions cannot be grown at that pH.

Rather than maintain a list of banned vegetables, judge each crop by two questions: does it need more nutrients than my fish can supply, and can it grow at a near-neutral pH?

According to NMSU Circular 680, the optimal compromise for fish, plants and nitrifying bacteria together is pH 6.8 to 7.0, and anything held between 6.4 and 7.4 is tolerable to all three. The competing preferences: plants grow best below 6.5, nitrifying bacteria perform best above 7.5 and stall below 6, and tilapia tolerate a very wide 5.0 to 10.0.

Test pH every day when starting out — it normally declines as nitrification produces acid. If it drops below 6.4, add a base such as calcium hydroxide or potassium hydroxide to bring it back to 7.0, alternating the two because both calcium and potassium may need supplementing. NMSU warns: if you skip testing and the pH crashes (as low as 4.5 is possible), raise it very slowly — dumping in a large amount of base at once can convert accumulated ammonia into its toxic form and kill the fish.

Plan on roughly a 5-week lettuce cycle: Cornell's home guide follows the commercial schedule of a 35-day growing cycle to a 5-ounce head of lettuce, transplanting seedlings into the system around day 11. Herb seedlings are generally ready to transplant after 2 to 3 weeks; a good visual cue is 3–4 true leaves. Cooler temperatures and lower light stretch all of these times.

You will see claims that plants grow "25–50% faster" or twice as fast as in soil. We cannot trace figures like those to a study, so we do not repeat them — judge the speed of your own system against the 35-day lettuce schedule above.

Yes. The two things to get right are light and water temperature:

• Light — for leafy greens Cornell's rule of thumb is 12–17 mol/m²/day of light; when you cannot measure that, about 12 watts per square foot of growing area is a workable target (a 2 ft × 2 ft area needs roughly 48 watts).
• Water temperature — choose your fish for the temperature you can hold: warmwater species prefer 65–85°F, coldwater species like trout thrive at 55–65°F (NMSU).

Watch the plants themselves: Cornell says stretched, thin seedlings want more light, and leaf tip burn can mean too much.

Feed daily, and only what the fish will actually take. The reason is chemistry, not habit: NMSU reports that one pound of feed produces 0.25 to 0.30 pounds of settleable solids — uneaten food and waste that consume oxygen, release ammonia as they decay, and clog pipes if they are not removed before the biofilter and plant roots.

Overfeeding is the first cause NMSU lists for ammonia problems, along with stocking too many fish for the water volume — their rule of thumb is 1 lb of fish per 2 gallons of water. If plants are growing poorly because there is not enough ammonia being produced, the fix is more fish, more feed, or a smaller tank.

In a running system, the nutrients most likely to need topping up are calcium, potassium and iron — NMSU singles these out as the elements fish feed does not reliably supply. Potassium and calcium come in with the pH-adjusting bases if you alternate them, and NMSU's iron fix is 2 mg/L of chelated iron every 3 weeks.

If the system is young, the problem is usually upstream of the leaves: not enough ammonia is being produced because there are too few fish for the number of plants. Add fish, feed more, or use less water (NMSU). Test your water before buying supplements.

Test the water immediately; NMSU Circular 680 gives the warning lines:

• Ammonia — keep total ammonia nitrogen below 1 ppm; more is produced when you overfeed, stock too densely, or aerate too little.
• Nitrite — toxic to fish at 5 ppm; hold it at or below 1 ppm, especially with tilapia.
• Oxygen — warmwater fish need about 5 ppm dissolved oxygen, trout about 6.5 ppm; tilapia gasping at the surface means it has fallen to around 1 ppm.
• Chlorine — municipal water is treated with chlorine and chloramines, which must be removed before the water goes to fish.

If fish are stressed, NMSU's advice is a water exchange: remove up to a third of the tank and replace it with fresh, dechlorinated water at the same temperature and pH as the tank water, so the change itself does not add stress.

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