MgSO4 Epsom Salt Aquarium Dosing Guide
Singapore tap water is so soft that magnesium often runs below 5 ppm out of the tap, and the planted tanks fed it eventually show the symptoms even when every other macro is dialled in. This mgso4 epsom salt aquarium dosing guide from Gensou Aquascaping at 5 Everton Park covers why magnesium matters, the simple Epsom salt recipes that fix it, and the targets that suit planted tanks, shrimp colonies and goldfish setups alike. Magnesium sulphate heptahydrate (Epsom salt) is the same compound found in foot-soak bags, costs almost nothing, and is the single most cost-effective dosing salt in the hobby.
Why Magnesium Matters for Plants
Magnesium sits at the centre of every chlorophyll molecule. Without enough Mg, plants cannot photosynthesise efficiently regardless of light, CO2 or other nutrients. Symptoms include interveinal chlorosis on older leaves where the leaf veins stay green but the tissue between them yellows. Stem plants like Ludwigia and Rotala reveal it within a week; slower-growing Anubias and Bucephalandra take a month. The aquarium magnesium deficiency plants piece has photo references.
The Chemistry of Epsom Salt
Epsom salt is magnesium sulphate heptahydrate (MgSO4·7H2O), 9.86 percent magnesium by weight. One gram dissolved in 100 litres of water raises Mg by approximately 1 ppm and SO4 by approximately 4 ppm. For a 60 litre tank wanting +5 ppm Mg, you need roughly 3 grams of Epsom salt. The sulphate contribution is biologically inert at these levels and adds negligibly to TDS for shrimp keepers.
Singapore Tap Water Baseline
PUB tap typically arrives at GH 2 to 4, with magnesium between 1 and 5 ppm and calcium between 10 and 20 ppm. The standard 3:1 calcium-to-magnesium ratio that planted tanks favour means tap water magnesium is almost always low. A weekly water change at 50 percent dilutes any Mg you have added back toward tap baseline, so dosing on water-change day matters more than mid-week top-ups.
Stock Solution Recipe
Dissolve 200 grams of Epsom salt in distilled or RO water to 500 ml total. The salt dissolves rapidly with stirring and produces a clear solution. Store in opaque PET; properly mixed Epsom solution keeps indefinitely without precipitation. Mark the bottle clearly because Epsom looks similar to other dosing salts when dry. From this 400 g/L stock, 1 ml delivers 0.4 grams of Epsom, raising Mg in a 60 litre tank by approximately 0.66 ppm.
Targets for Planted Tanks
Most planted tanks thrive at Mg between 5 and 15 ppm sustained. Heavy stem tanks running EI benefit from the higher end; shrimp-only and low-tech tanks do well at the lower end. For a typical CO2-injected 60 litre tank with stems, dose enough Epsom to maintain 10 ppm Mg, which works out to roughly 4.5 grams of Epsom per week for tap-baseline conditions. Pair this with the macro routine from the aquarium ei dosing complete guide.
Targets for Shrimp Tanks
Caridina shrimp using GH boosters like Salty Shrimp GH+ already get balanced calcium and magnesium in a 3:1 ratio. Adding Epsom on top is rarely needed and can throw off the ratio. For Neocaridina tanks fed PUB tap with no GH booster, supplemental Epsom helps moulting success; aim for 5 to 8 ppm Mg. The aquarium tds explained for shrimp keepers piece covers the TDS implications of any salt addition in shrimp systems.
Targets for Goldfish and Africans
Hard-water fish thrive at higher magnesium levels. Goldfish, mbuna and Tanganyikan keepers often dose Epsom to support 15 to 25 ppm Mg alongside calcium-rich substrates. Higher levels also support fin and scale development. Avoid the temptation to dose to seawater levels (around 1,300 ppm); freshwater fish do not need that and excess sulphate at extremes can stress kidneys.
Therapeutic Use for Constipation
Epsom salt has long been used as a laxative for constipated fish, particularly goldfish, fancy bettas and pufferfish. The therapeutic dose is roughly 1 tablespoon (15 grams) per 20 litres in a hospital tank, dissolved fully before the fish enters. The osmotic effect draws water through the gut and relieves blockages. Use distilled water for the hospital tank, not tap, to avoid chloramine complications. Limit treatment to 24 to 48 hours.
Where to Buy in Singapore
Watson’s, Guardian and Unity stock food-grade Epsom salt for foot soaks at roughly $4 per kilogram. Pasar Bella and Cold Storage sell larger bags at $6 to $8 per kilogram. Aquarium-branded “magnesium supplement” pouches at C328 cost $12 to $18 per 250 grams; the chemistry is identical and the markup is hard to justify. Industrial-grade Epsom from Shopee at $5 per kilogram is fine if labelled food-grade or USP-grade.
Dosing Schedule for a 60 Litre Planted Tank
For a CO2-injected 60 litre tank, dose 1.5 grams of Epsom (3.75 ml of 400 g/L stock) on water-change Sunday plus 0.5 grams on Wednesday. That delivers approximately 3.3 ppm Mg per Sunday dose and 1.1 ppm midweek, maintaining 10 to 12 ppm Mg before water-change reset. Combine with KNO3 and KH2PO4 on Mon/Wed/Fri and traces on Tue/Thu/Sat. Pair the schedule with the structure in our aquarium water change guide.
Interactions and Diagnostic Pitfalls
Magnesium does not precipitate with phosphate at typical planted tank levels, so you can dose Epsom and KH2PO4 together. Calcium, however, can interact: Ca:Mg ratios skewed too far either direction reduce uptake of the deficient ion. Maintain calcium at roughly 3x magnesium in soft-water tanks. The aquarium calcium magnesium guide walks through the ratio mechanics.
If interveinal chlorosis continues after two weeks of correct Epsom dosing, verify your test kit reads true. Cheap GH kits do not differentiate Mg from Ca, so a “GH 6” reading might be all calcium with no magnesium. A dedicated Mg test kit (Salifert or Hanna) gives the actual reading. If Mg tests at target but symptoms persist, check iron and manganese, which produce similar interveinal patterns. The aquarium plant deficiency guide covers the differential diagnosis.
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