Inputs · Nutrients and supplements
Seaweed or kelp extract
Seaweed is the one organic supplement the FAO aquaponics manual names as rich in potassium and iron, the two nutrients aquaponic systems most often lack, and it recommends a home-brewed seaweed tea. Bottled kelp and seaweed extracts are popular with aquaponic growers, but we found no UF/IFAS, SRAC, FAO or extension document that tests or recommends them; their iron and potassium claims come from sellers.1
Why you need it1
FAO says fish feed is generally low in iron, calcium and potassium, so even balanced systems can run short, especially with fruiting crops or in cold months. It names compost tea and seaweed tea as aquaponics-safe supplements, says seaweed is a great addition because it is rich in potassium and iron, and lists seaweed tea as a way to supplement iron in a new system alongside chelated iron.
How to use it1
FAO's method for nutrient teas, which it says applies to seaweed as well as compost: tie several handfuls of the material in a mesh bag weighted with stones, hang it in a bucket of about 20 litres of water, put an air stone underneath so the bubbles keep it agitated, and stir now and then; after 2 to 3 days strain it through a fine cloth and dilute it 1 part tea to 10 parts water. Use it as a foliar spray or pour it straight into the root zone. If adding it to the system water, start small (about 50 ml), record how the plants respond, and repeat when needed. Rinse seaweed first to remove residual salt. FAO also says larger amounts of such teas can keep plants fed temporarily when fish are out of the system, for example in cold months.
Cautions1, 2
Salt: FAO says to rinse residual salt off seaweed, and SRAC 454 says sodium above 50 mg/L interferes with potassium and calcium uptake and causes tip burn. Aeration matters: FAO says to keep teas constantly aerated to prevent anaerobic fermentation and some pathogenic bacteria. FAO also warns against getting aquaponic water on plant leaves, especially when using compost tea, which sits awkwardly with its own foliar-spray suggestion (see gaps). For bottled products, no fish-safety or plant-response data were found in any fetched non-seller source.
What to buy1
For the method FAO documents, you need dried or fresh seaweed, a mesh bag, a bucket, an air pump and an air stone. For bottled kelp or seaweed extracts the practice is popular but undocumented: we found no extension, SRAC or FAO guidance on what to look for on the label, how much to use, or whether the products are safe with fish. Treat their iron and potassium figures as sellers' claims.
What this page doesn't cover yet
- Commercial kelp or seaweed extracts: no UF/IFAS EDIS, SRAC, FAO or university extension source was found that tests or recommends them in aquaponics. Search results turned up sellers' pages with iron and potassium analyses and general crop biostimulant reviews (field crops, not aquaponics); none were used as a source.
- FAO gives no iron or potassium figures for seaweed tea, and no dose beyond 'start with 50 ml and observe'.
- FAO contradicts itself slightly: it suggests teas as a foliar spray, and also says to always avoid placing aquaponic water on leaves, especially with compost tea. A reviewer may want to resolve which to lead with.
- Florida-specific: no source covers collecting seaweed from Florida beaches (rules, contaminants); florida_note left null.