Reef Magnesium Imbalance Fix Guide: Low and High Correction
Magnesium is the parameter most reefers underestimate until alkalinity and calcium both stop responding to dosing. A reef magnesium imbalance at 1180 ppm or 1450 ppm produces secondary effects so confusing — drifting alkalinity, stalled coralline growth, browning frags — that keepers chase the wrong fix for weeks. Built from twenty years of reef chemistry at Gensou Aquascaping at 5 Everton Park, this guide explains how to detect the imbalance, the safe correction window, and the products that work in Singapore.
Why Magnesium Anchors Reef Chemistry
Magnesium prevents calcium and carbonate from precipitating out as calcium carbonate. At normal seawater levels of 1280-1350 ppm, the precipitation rate stays low and dosed alkalinity and calcium remain bioavailable. Below 1250 ppm, precipitation accelerates and your dosing seemingly disappears. Above 1400 ppm, calcium gets sequestered and coral skeleton growth slows.
Testing Magnesium Properly
Salifert Magnesium Profi-Test is the hobby reference at SGD 38 per kit, accurate to ±20 ppm. Red Sea Magnesium Pro is faster at the cost of broader confidence intervals. ICP testing through ATI or Triton catches drift earlier. Test monthly for stable reefs, fortnightly during dosing changes. Stock the full magnesium test range in the water care range.
Symptoms of Low Magnesium
Alkalinity drops faster than expected despite consistent dosing. Calcium readings drift downward in parallel. Coralline algae growth pauses. Bryopsis and hair algae become more aggressive — magnesium suppression of these algae is one of fluconazole’s mechanisms of action. The pattern is unmistakable: dosing cannot keep up despite no change in stocking or feeding.
Symptoms of High Magnesium
Calcium drops despite calcium dosing because excess magnesium inhibits calcium uptake. SPS coral growth slows or stalls. New coralline patches do not form. The cause is often over-correction during a previous low-magnesium event, or accidentally dosing magnesium when the test reading was wrong. High magnesium is rarer but harder to correct than low.
Correction Rate for Low Magnesium
Raise magnesium no faster than 50 ppm per 24 hours to avoid stress on existing inhabitants. From 1180 to 1320 ppm — a 140 ppm correction — schedule across three days minimum. Tropic Marin Bio-Magnesium and Aquaforest Magnesium are the safest products. Brightwell Magnesion-P is the dry alternative for cost-conscious reefers. Calculate dose using the manufacturer’s per-litre table on your specific water volume.
Correction Rate for High Magnesium
Reducing magnesium requires water changes with reef-grade salt that mixes to natural seawater levels. Tropic Marin Pro Reef mixes at 1330 ppm magnesium; Red Sea Coral Pro at 1340 ppm. A 10 per cent water change with new salt drops magnesium by roughly 10 per cent of the difference between current and target. From 1450 ppm down to 1330 ppm typically takes three to four 10 per cent changes spaced weekly.
Why Salt Mix Matters
Reef-grade salts mix to specific magnesium targets — Tropic Marin Pro Reef 1330 ppm, Red Sea Coral Pro 1340 ppm, Aquaforest Reef Salt 1350 ppm. Generic salts often mix lower at 1250-1280 ppm, dragging tank magnesium downward over months of water changes. Test your freshly-mixed salt before use to verify the actual magnesium level matches the label.
Magnesium and Trace Elements
Magnesium dosing affects boron, sulphate and strontium balance. After a major magnesium correction, ICP through ATI Coral Lab or Triton catches secondary drift before it manifests as colour loss. Browse ICP sample bottles in the aquarium equipment range. Plan ICP six weeks after any major magnesium swing.
Routine Maintenance Dose
A mature 200-litre mixed reef typically consumes 0.5-1.5 ppm magnesium per day. Two-part dosers usually do not include magnesium, so manual weekly top-up of 5-10 ml of liquid Bio-Magnesium per 100 litres holds the reading steady. SPS-heavy systems may need automated magnesium dosing through a third channel. Trickle dosing is gentler than once-weekly bolus.
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