After heavy rain, many farmers test their water, see magnesium, potassium or alkalinity below “the table”, and immediately ask: how much should I add? This article shows how to calculate the amount of minerals your own pond needs with one easy rule of thumb. Just as important: knowing how far to correct is enough, so you do not waste chemicals.
This is part of our El Niño 2026–2027 series. See also Part 2 – the 9 buffers: minerals in the water and minerals in the feed are the two buffers that wear down fastest in the rainy season.
1. Measure first, then correct
Within 24–48 hours after heavy rain, re-check DO, temperature, pH, alkalinity and salinity. In low-salinity ponds or ponds using inland water, also measure Ca, Mg and K, and if possible Na to assess the Na:K ratio.
This step matters because two ponds at the same salinity can have very different ion profiles. Without measurements, every calculation below is just a guess.
2. Rule of thumb: 1 mg/L × 1,000 m³ = 1 kg
1,000 m³ of water is one million litres. Raising an element (Ca, Mg or K) by 1 mg/L in 1,000 m³ takes exactly 1 kg of that element. Divide by the element content of the chemical to get the amount of chemical needed:
Theoretical example with pure chemicals, to raise 10 mg/L in 1,000 m³:
| Chemical | Element content | Amount needed |
|---|---|---|
| MgCl₂·6H₂O (magnesium chloride) | 12% Mg | ≈ 84 kg |
| MgSO₄·7H₂O (magnesium sulfate) | 9.9% Mg | ≈ 101 kg |
| KCl (potassium chloride, muriate of potash) | ≈ 50% K | ≈ 20 kg |
| CaCl₂·2H₂O (calcium chloride) | 27% Ca | ≈ 37 kg |
| NaHCO₃ (sodium bicarbonate), +10 mg/L alkalinity as CaCO₃ | — | ≈ 16.8–17 kg |
For alkalinity, raising 1 mg/L (as CaCO₃) in 1,000 m³ takes about 1.68 kg of pure NaHCO₃. In practice, calculate from the COA or the actual content of the raw material, since commercial products are not always 100% pure.
3. How much is enough? Enough means what shrimp need
Diluted-seawater levels at the same salinity (Ca 11.6, Mg 39.1, K 10.7 mg/L per ppt) are only a reference, not a mandatory threshold. Pulling a pond up to them usually takes a lot of chemical for no benefit. What shrimp need, based on published trials:
| Item | Practical target | Basis |
|---|---|---|
| K | Seawater Na:K (≈ 28:1), but no more than 50 mg/L | At 4 ppt, growth rose with K up to 40 mg/L (Roy et al., 2007); in 4 ppt well water, Na:K 23–33:1 improved survival and growth (Liu et al., 2014) |
| Mg | Not below Ca, at least 30 mg/L | Mg 30 mg/L with Mg:Ca 1:1 did not reduce growth at 5 ppt (Aquaculture, 2023); Mg 10 mg/L gave only 60% survival (Roy et al., 2007) |
| Ca | Level with K (Ca:K ≈ 1:1 as in seawater), at least 30 mg/L; 50 mg/L from 5 ppt up | SRAC 2601 (Davis, Samocha & Boyd); after molting, shrimp take Ca from the water to harden the shell |
| Alkalinity | At least 80, best 100–120 mg/L as CaCO₃; below 80, step 1 only raises it to 80 | Boyd (Global Seafood Alliance); raise in steps so the water does not change too fast |
For example at 5 ppt: the seawater reference for K is about 53 mg/L, but the practical target is capped at 50 mg/L.
4. Example: a 3,000 m² pond, 1.2 m deep
The pond holds about 3,000 × 1.2 = 3,600 m³. After rain the readings are: salinity 5 ppt, Mg 160 mg/L, K 45 mg/L, Ca 62 mg/L, alkalinity 70 mg/L. Compared with what shrimp need, only K and alkalinity are short:
| Item | Working | Result |
|---|---|---|
| K 45 / 50 → short 5 | 5 × 3.6 = 18 kg K ÷ 50% K (KCl, muriate of potash) | ≈ 36 kg KCl, split into 2 × 18 kg |
| Mg 160 / ≥ 62 (equal to Ca) → enough | — | No need to add |
| Ca 62 / ≥ 50 (level with K) → enough | — | No need to add |
| Alkalinity 70 / ≥ 80 → step 1 to 80 | 10 × 3.6 × 1.68 (99% NaHCO₃) | ≈ 61 kg NaHCO₃ for step 1; re-test before raising to 100 (another ≈ 122 kg) |
So this pond needs only about 36 kg of KCl (in two doses) and 61 kg of sodium bicarbonate for the first step. Correcting what is actually short, up to what shrimp need, is enough — there is no need to match seawater.
To calculate for your own pond, enter your readings in the mineral calculator by salinity: it gives the kilograms to apply, how to split them, why some ions need no correction, and lets you share the result.
5. Split doses – mix separately – re-measure
- Split doses: for large amounts, dose in several steps and re-measure after each large dose before deciding the next one.
- Timing: there is no single fixed hour for every pond. What matters is that DO is good, aerators are running, shrimp are not short of oxygen and the environment is no longer swinging.
- Mix each chemical separately: do not dissolve CaCl₂ in the same tank as NaHCO₃ or other carbonates. Ca²⁺ can react with carbonate/bicarbonate and precipitate, reducing the effect.
6. Magnesium costs the most, so measure first
When pushing low-salinity water toward seawater ion ratios, Mg usually needs the largest amount of chemical, especially as MgCl₂·6H₂O or MgSO₄·7H₂O. So instead of “adding minerals out of habit after rain”:
The more minerals cost, the more testing pays off.
7. What about mineral and trace-element products?
When shrimp have soft shells or need mineral and trace-element support, products such as CELL CALPHOS GOLD, MIXBOOM and VIMIX should be used according to the current label and the actual pond condition, not as a substitute for measuring Ca, Mg and K. Shrimp need only small amounts of trace elements (Cu, Zn, Fe); use the label dose and do not increase it on your own.
Bottom line: stability first, correction second
There is no “magic number” for salinity, alkalinity, Ca, Mg or K. Shrimp tolerate a fairly wide range when change is gradual; when many factors swing quickly, their ability to adapt drops. So:
- Do not add minerals just because it rained.
- Do not chase every number that is “below the table” at any cost.
- Do not wait for soft shells before thinking about minerals.
MEASURE FIRST – CORRECT RIGHT – SPLIT DOSES – RE-MEASURE. That is the safer and more economical way to keep minerals, and keep the pond.
References: Roy L.A. et al. (2007), Aquaculture 262: 461–469 – aqueous potassium and magnesium in low-salinity water; Liu H. et al. (2014), Chinese Journal of Oceanology and Limnology 32(5): 991–999 – Na/K ratios in low-salinity well water; Aquaculture (2023) – magnesium reduction in artificial low-salinity water for Pacific white shrimp in biofloc (5 g/L); Davis, Samocha & Boyd (2004), SRAC 2601; Boyd C.E., Water composition and shrimp pond management (Global Seafood Alliance); element contents calculated from chemical formulas.
