Aquaponics FL

Inputs · Nutrients and supplements

Calcium

Calcium is usually short in aquaponic water, and lettuce tip burn and tomato blossom-end rot are the classic signs. Like potassium, it goes in as a base, either calcium hydroxide (fast) or calcium carbonate (slow), so it also offsets the acid from nitrification. In much of Florida, hard well water brings some calcium in with every top-up.1, 2

Why you need it1, 2, 4

SRAC 454 says calcium and potassium are generally insufficient in aquaponic water; FAO and the UF/IFAS guide HS1252 list calcium with potassium and iron as the nutrients fish feed does not supply in adequate amounts. FAO describes deficiency as always showing in the newest growth, because calcium does not move within the plant: lettuce tip burn, blossom-end rot in tomatoes and zucchini, and new leaves that come out distorted with hooked tips. Calcium can also be present but locked out: it travels only with the water a plant transpires, so in very humid conditions plants stop taking it up, and FAO suggests more air movement from vents or fans. FAO adds that high water temperature restricts calcium uptake, and SRAC 454 says sodium above 50 mg/L interferes with calcium uptake and causes tip burn, most often in warm months.

How to use it1, 2, 3, 4

Two forms. Calcium hydroxide (hydrated lime): SRAC 454's UVI system alternates it with potassium hydroxide in equal amounts several times a week to hold pH near 7.0, and HS1252 lists it for raising pH. FAO says it is a strong base, to be added slowly like an acid. Calcium carbonate: FAO calls it the safer, easier option, raising both KH and pH, and lists cheap sources: crushed eggshells, finely crushed seashells, coarse limestone grit, crushed chalk and coral sand. Put the material in a porous bag hung in the sump, keep testing pH over the following weeks and remove the bag if pH climbs above 7; or add 2 to 3 handfuls per 1,000 litres straight into the media beds or biofilter. Rinse seashells first to remove salt. HS1252 says limestone (and dolomite) gives a slower rise in pH and a longer-lasting buffer than the hydroxides. SRAC 452 warns calcium carbonate may dissolve too slowly to keep up with a rapid build-up of acid. Gravel media can supply calcium on its own, depending on what it is made of, as it reacts with the acid from nitrification (SRAC 454). FAO suggests calcium bases for leafy greens, to head off tip burn.

In Florida7, 6, 5, 2

Florida groundwater is hard: UF/IFAS Marion County Extension explains that the state's landmass formed from calcium-carbonate-rich marine deposits and that hardness ions are present throughout Florida groundwater; UF/IFAS PI245 says most of the state's groundwater sits in the limestone Floridan aquifer; and UF/IFAS SL219 says much Florida well water is alkaline (pH 7.6 to 8.5) from contact with that calcium carbonate rock. FAO says hard water is a useful source of calcium and magnesium for plants and that topping up with it also replenishes the system's buffer. A well test from the UF/IFAS Extension Soil Testing Laboratory in Gainesville reports calcium and magnesium (SL219).

Cautions1, 3, 2

Too much calcium can make phosphorus drop out of the water as dicalcium phosphate (SRAC 454), and excess calcium interferes with potassium and magnesium uptake (SRAC 454). Calcium hydroxide and calcium oxide dissolve fast and are very caustic; SRAC 452 says not to add them to the rearing tank, because they create zones of very high pH that harm fish. If you use a bag of shell or limestone, take it out once pH passes 7 (FAO).

What to buy2

Calcium hydroxide is sold as hydrated lime (FAO's appendix uses that name). For calcium carbonate, FAO lists eggshells, seashells, limestone grit, chalk and coral sand; rinse anything from the beach. No source names brands or grades.

What this page doesn't cover yet

  • No source gives a target calcium concentration in mg/L for aquaponics, or a home-scale dose of calcium hydroxide.
  • No source gives measured hardness or calcium figures for Florida wells by region; the Florida note is qualitative.
  • Calcium nitrate and calcium chloride, both common hydroponic calcium sources, are not mentioned in any fetched aquaponics source.

Sources

  1. Recirculating Aquaculture Tank Production Systems: Aquaponics—Integrating Fish and Plant Culture. Southern Regional Aquaculture Center (SRAC), SRAC 454. Read 2026-11-02.
  2. 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.
  3. Recirculating Aquaculture Tank Production Systems: Management of Recirculating Systems. Southern Regional Aquaculture Center (SRAC), SRAC 452. Read 2026-11-02.
  4. A Practical Guide for Aquaponics as an Alternative Enterprise (Tyson and Simonne 2014), archival copy. UF/IFAS Extension, EDIS, HS1252. Read 2026-11-02.
  5. Irrigation and Household Water Test and Interpretation (Mylavarapu). UF/IFAS Extension, EDIS, SL219. Read 2026-11-02.
  6. Water Quality and the Effectiveness of Pesticides (Fishel). UF/IFAS Extension, EDIS, PI245. Read 2026-11-02.
  7. Water Quality Series: Hard Water (Vicari 2021). UF/IFAS Extension Marion County. 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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