Aquaponics FL

Inputs · Nutrients and supplements

Magnesium

Magnesium sits at the centre of the chlorophyll molecule, so a shortage shows as yellowing leaves. The sources disagree on how often aquaponic systems run short: FAO says adding it is generally unnecessary, while SRAC 454 says it may be limiting in some systems. Where it is needed, the documented way to add it is dolomite used as a pH buffer.2, 1

Why you need it2, 1

FAO describes magnesium deficiency as yellowing between the leaf veins, especially in older parts of the plant. FAO says magnesium is sometimes low in aquaponics but does not appear to limit growth; SRAC 454 says that in some systems it may be limiting.

How to use it1, 3, 2

SRAC 454: use dolomite, a calcium magnesium carbonate, as the base that adjusts pH, so magnesium goes in with calcium as acid is neutralised. The UF/IFAS guide HS1252 says dolomite (like limestone) gives a slower rise in pH and a longer-lasting buffer than the hydroxide bases. FAO's general method for carbonate buffers applies: a porous bag in the sump, removed if pH rises above 7 (see Calcium).

In Florida2, 5, 4

Hard water carries magnesium. FAO defines general hardness as mostly calcium and magnesium ions, both plant nutrients taken up as water flows past the roots. UF/IFAS Marion County Extension says these hardness ions are present throughout Florida groundwater, and the UF/IFAS Extension Soil Testing Laboratory well test reports magnesium, which it uses with calcium to calculate hardness (SL219). Test before adding any.

Cautions1, 2

Nutrients compete: SRAC 454 says excess potassium can interfere with magnesium and calcium uptake, and excess magnesium or calcium with the other two. Adding a lot of potassium hydroxide or calcium can therefore bring on a magnesium shortage. Epsom salt (magnesium sulfate) is widely used by gardeners, but no fetched aquaponics source gives a dose or recommends it; FAO mentions magnesium sulfate only once, as a fish-disease remedy in its appendix.

What to buy1, 3

If a source-water test and leaf symptoms point to magnesium, the documented product is dolomite (calcium magnesium carbonate) used as the pH buffer (SRAC 454, HS1252). No source names grades or brands.

What this page doesn't cover yet

  • FAO (generally unnecessary) and SRAC 454 (may be limiting in some systems) disagree on whether magnesium needs adding.
  • Epsom salt (magnesium sulfate): no UF/IFAS, SRAC, FAO or extension aquaponics source found that gives a rate; a single small foliar-spray study on beetroot turned up in search but was not opened. Listed as an alias only so searches land here and readers see the caution.
  • No source gives a target magnesium concentration for aquaponics.

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. 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. Irrigation and Household Water Test and Interpretation (Mylavarapu). UF/IFAS Extension, EDIS, SL219. Read 2026-11-02.
  5. 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.

Need magnesium or a test kit?

Tell us where you are and what you're after; we pass it to Florida aquaponics suppliers. No cost to you.