Inputs · Nutrients and supplements
Potassium is one of three nutrients (with calcium and iron) that fish feed does not supply in the amounts vegetables need. The sources add it in a form that also raises pH, so one addition both feeds the plants and counters the acid that nitrification makes. Fruiting crops such as tomatoes and peppers need it most.1, 2, 3
SRAC 454 says potassium and calcium levels are generally insufficient in aquaponic water while nitrate, phosphate and sulfate are usually fine; FAO and the UF/IFAS guide HS1252 say the same of potassium, calcium and iron. FAO describes the deficiency as burnt-looking spots on older leaves, poor vigour and limp plants, with yellowing between the veins near the leaf edges; without enough potassium, flowers and fruit do not develop properly. The Fort Valley State extension slides describe yellowing on older leaves, then dead leaves.
Which salts the sources recommend: potassium hydroxide (KOH) in SRAC 454, HS1252, FAO and the Fort Valley slides; potassium carbonate (K2CO3) in FAO, which calls carbonates a safer and easier way to raise pH than hydroxides because they raise carbonate hardness (KH) as well; and potassium bicarbonate, which FAO's appendix lists as a way to raise KH and buffer pH. How they double as a buffer: nitrification makes acid, and adding these bases neutralises it while supplying potassium (SRAC 454). The UVI commercial system in SRAC 454 adds potassium hydroxide and calcium hydroxide in equal amounts (usually 500 to 1,000 g per addition at that commercial scale), alternating them several times a week to hold pH near 7.0; the Fort Valley slides describe the same alternation at pH 7.0 to 7.5. Choose by crop: FAO suggests potassium bases for fruiting plants (flowering, fruit set and ripening) and calcium bases for leafy greens. Add slowly: FAO says hydroxides are strong and should be added the way acids are, changing pH gradually. SRAC 454 adds base in the sump, or in a separate aerated base-addition tank that drains into the sump, so the high-pH water is diluted before it reaches the fish.
UF/IFAS (SL219) says much of Florida's well water is already alkaline, pH 7.6 to 8.5, from the limestone aquifer, and FAO says hard, alkaline source water can mean acid rather than base is needed. Our reading, not a source statement: a Florida system topped up from such a well may rarely need a pH-raising base, so potassium may not arrive by that route; no fetched source names a potassium supplement that does not also raise pH (see gaps).
Too much potassium can block magnesium and calcium uptake (SRAC 454). Potassium hydroxide makes a zone of high, toxic pH where it dissolves (SRAC 454); SRAC 452 gives the same warning about caustic bases in general and says not to add them to the rearing tank. Never use sodium bicarbonate (baking soda) as a substitute: SRAC 454, FAO and the Fort Valley slides all say the sodium builds up and harms plants. Wear eye protection and gloves when handling strong bases, as FAO advises for acids and says to treat bases the same way.
Look for the chemical name on the label: potassium hydroxide, potassium carbonate or potassium bicarbonate. FAO's appendix lists potassium bicarbonate as acceptable in aquaponics (as a foliar spray and to raise KH). No source names brands or grades for potassium salts.
Restated in our own words from these documents, written 2026-11-02. Doses are the sources' figures; test your water before and after.