Aquaponics FL

Inputs · pH

pH adjusters (acids, bases and buffers)

Aquaponic water drifts acid on its own, because the bacteria that turn ammonia into nitrate release acid. So most systems need a base now and then, and the sources say to use potassium or calcium bases, which feed the plants at the same time. Acid is needed only where the top-up water is very hard and alkaline, which describes much Florida well water, and it has to be added with great care.1, 2

Why you need it1, 4, 8, 3, 2, 7

pH sets how well everything works. Plants reach all nutrients at pH 6.0 to 6.5, and pH 7.5 can cause iron, phosphorus and manganese shortages (FAO). Nitrifying bacteria struggle below pH 6 (FAO; SRAC 452), and SRAC 451 says activity drops below pH 7.0. Most aquaponic fish tolerate pH 6.0 to 8.5 (FAO), but higher pH makes ammonia more toxic: the UF/IFAS guide HS1252 says that at 82 °F the toxic form is 0.2, 2 and 18 percent of total ammonia at pH 6.5, 7.5 and 8.5. Targets differ by source: FAO says 6 to 7; SRAC 454 says close to 7.0 as the compromise between nitrification and nutrient uptake; HS1252 says 6.5 to 7.5; the Fort Valley State extension slides say about 7.0. FAO says pH between 5.5 and 7.5 needs slow, measured correction, and below 5 or above 8 needs immediate attention.

How to use it1, 2, 3, 4, 7

Raising pH (the usual job). The documented bases are potassium hydroxide and calcium hydroxide (SRAC 454, FAO, HS1252, Fort Valley slides), which SRAC 454's UVI system alternates several times a week to hold pH near 7.0; carbonates, which FAO calls safer and easier because they raise carbonate hardness (KH) as well as pH: calcium carbonate (eggshells, crushed seashells, limestone grit, chalk) or potassium carbonate; potassium bicarbonate (FAO appendix); and dolomite, which adds magnesium (SRAC 454) and gives a slower, longer-lasting rise (HS1252). Choose by crop: calcium for leafy greens, potassium for fruiting plants (FAO). Add strong bases slowly, into the sump or a separate aerated base-addition tank so the water is diluted before it reaches fish (SRAC 454); HS1252 says to mix any fast-acting pH adjuster into a separate small tank and feed it in slowly. Lowering pH (rarely needed). FAO says to be patient first, since the system acidifies itself, and to add acid only when the source water is very hard and alkaline. Then treat a reservoir of top-up water, not the system itself, so that a mistake cannot crash the system. Phosphoric acid is FAO's choice: relatively mild, sold food-grade by hydroponic and farm suppliers, though overuse builds up phosphorus. Sulfuric acid has been used for extremely hard water but is too dangerous for beginners; nitric acid has also been used (FAO). Always add acid to water, never water to acid, and wear goggles and gloves (FAO). Test weekly at least: FAO's weekly tests are pH, nitrate, KH and temperature, and SRAC 452 says monitor pH daily in recirculating systems.

In Florida6, 1

UF/IFAS SL219 says much of Florida's well water is alkaline, pH 7.6 to 8.5, from the limestone (calcium carbonate) aquifer, and that acid treatment can neutralise its liming effect. FAO says water from limestone wells typically carries 150 to 180 mg/L of carbonate hardness, which buffers against the system's natural acidification, and that hard, basic top-up water combined with high evaporation is the main case where acid is needed. Our reading of those two sources together: Florida growers on well water are more likely than most to face high pH, and FAO's advice for that case is to acid-treat the top-up water in a separate reservoir rather than dose the system.

Cautions2, 1, 7, 4, 5, 3

Never use baking soda (sodium bicarbonate) in aquaponics: it is a standard buffer in fish-only systems (SRAC 452, SRAC 4502), but SRAC 454, FAO and the Fort Valley slides all say the sodium builds up and harms plants. Do not use citric acid: FAO says it is antimicrobial and can kill the biofilter bacteria (HS1252 lists citric acid among its pH-lowering options; see gaps). Acid danger: the water's buffers absorb the first doses with no visible change, then pH crashes all at once when the buffer runs out (FAO). Calcium hydroxide, calcium oxide and sodium hydroxide are caustic and must not go straight into the rearing tank (SRAC 452). Change pH slowly in either direction (FAO).

What to buy1

For raising pH, look for potassium hydroxide, calcium hydroxide (hydrated lime), potassium carbonate, potassium bicarbonate, calcium carbonate or dolomite on the label. For lowering pH, FAO recommends food-grade phosphoric acid from a hydroponic or agricultural supplier. Avoid products based on sodium bicarbonate or citric acid. A pH test (FAO: colour-coded kits, strips, or a digital meter, the last being more expensive and more accurate) and a KH test are needed to use any of these.

What this page doesn't cover yet

  • Citric acid: HS1252 (archival UF/IFAS) lists it with sulfuric, phosphoric and hydrochloric acid for lowering pH; FAO says never use it because it kills biofilter bacteria. The page follows FAO; a reviewer should confirm.
  • Hydrochloric acid appears only in HS1252; FAO and SRAC do not mention it, so the page does not recommend it.
  • Target pH differs: FAO 6 to 7, SRAC 454 about 7.0, HS1252 6.5 to 7.5, Fort Valley slides 7.0 (with fish optimum 7.5). SRAC 4502 prefers 6.8 to 7.2 during start-up.
  • Lowest safe pH for nitrifying bacteria differs: FAO and SRAC 452 say they are inhibited below 6; SRAC 451 says activity drops below 7.0.
  • No source gives home-scale doses for any base or acid; the Florida note's 'more likely to face high pH' is our reading of SL219 and FAO together.

Sources

  1. Small-scale aquaponic food production: Integrated fish and plant farming (Somerville et al. 2014). Food and Agriculture Organization of the United Nations (FAO), FAO Fisheries and Aquaculture Technical Paper 589. Read 2026-11-02.
  2. Recirculating Aquaculture Tank Production Systems: Aquaponics—Integrating Fish and Plant Culture. Southern Regional Aquaculture Center (SRAC), SRAC 454. Read 2026-11-02.
  3. A Practical Guide for Aquaponics as an Alternative Enterprise (Tyson and Simonne 2014), archival copy. UF/IFAS Extension, EDIS, HS1252. Read 2026-11-02.
  4. Recirculating Aquaculture Tank Production Systems: Management of Recirculating Systems. Southern Regional Aquaculture Center (SRAC), SRAC 452. Read 2026-11-02.
  5. How to Start a Biofilter (DeLong and Losordo 2012). Southern Regional Aquaculture Center (SRAC), SRAC 4502. Read 2026-11-02.
  6. Irrigation and Household Water Test and Interpretation (Mylavarapu). UF/IFAS Extension, EDIS, SL219. Read 2026-11-02.
  7. Introduction to Aquaponics Systems and Management (Duncan), slide presentation. Fort Valley State University Cooperative Extension Program (hosted by Texas A&M AgriLife Extension). Read 2026-11-02.
  8. Recirculating Aquaculture Tank Production Systems: An Overview of Critical Considerations. Southern Regional Aquaculture Center (SRAC), SRAC 451. Read 2026-11-02.

Restated in our own words from these documents, written 2026-11-02. Doses are the sources' figures; test your water before and after.

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