K2SO4 Potassium Sulphate Dosing Guide: Potassium Supplement

When your nitrate and phosphate readings sit in target range but plants still show pinhole damage on older leaves, you have a potassium deficiency the rest of your dosing cannot fix. This k2so4 potassium sulphate dosing guide from Gensou Aquascaping at 5 Everton Park covers when to add standalone potassium, the recipe maths in soft Singapore tap water, and the situations where K2SO4 quietly outperforms KCl or potassium-bearing macros. K2SO4 is the cleanest way to push potassium without dragging nitrate, phosphate or chloride along for the ride.

Why Standalone Potassium Sometimes Matters

KNO3 and KH2PO4 both contribute potassium alongside their primary nutrients, and for many planted tanks that combined K input is sufficient. Heavy stem-plant tanks running EI typically pull more potassium than the bundled doses provide, and lean dosing schedules often hit nitrogen targets long before potassium targets. Pinhole damage on older leaves of Rotala rotundifolia, Ludwigia repens and Hygrophila species is the classic sign you need standalone K.

Chemistry and Contribution Maths

Potassium sulphate (K2SO4) is 44.9 percent potassium by weight, the highest K content of common dosing salts. One gram dissolved in 100 litres raises K by approximately 4.5 ppm. The sulphate ion is biologically inert at typical doses and contributes only a few ppm to TDS. For a 60 litre tank wanting +5 ppm K per dose, you need approximately 0.7 grams; for stock solutions, 100 grams in 500 ml of distilled water gives a 200 g/L stock where 1 ml delivers 0.2 grams.

K2SO4 vs KCl as Potassium Sources

Potassium chloride (KCl) is cheaper and commonly sold as a salt substitute at supermarkets, but the chloride ion accumulates in closed systems and stresses freshwater shrimp at sustained levels above 30 ppm. K2SO4 has no such accumulation problem. For shrimp tanks the choice is obvious; for fish-only planted tanks both work, but K2SO4 is cleaner long-term. The aquarium potassium dosing guide covers the broader source comparison.

Stock Solution Setup

Dissolve 100 grams of K2SO4 in distilled or RO water to 500 ml total. K2SO4 dissolves more slowly than KNO3 and benefits from gentle warming or extended stirring. A magnetic stirrer helps but is not essential; ten minutes of vigorous shaking achieves full dissolution at room temperature. Store in opaque PET out of direct sunlight; properly mixed K2SO4 keeps for at least six months. Mark the bottle clearly because dry K2SO4 looks identical to KNO3 and the wrong dose causes real problems.

Calculating the Standalone K Dose

Start by measuring K input from your existing KNO3 and KH2PO4 doses. A typical EI 60 litre tank dosing 4 g KNO3 and 0.6 g KH2PO4 weekly delivers roughly 26 ppm K from KNO3 and 3 ppm K from KH2PO4, totalling 29 ppm K weekly. If target is 30 to 40 ppm K per week, the standalone K2SO4 contribution should be 5 to 10 ppm. That works out to 0.7 to 1.4 grams per week split across three doses.

Symptoms of Potassium Deficiency

Pinhole damage on older leaves is the textbook sign. Holes start as small chlorotic spots, then dissolve into ragged perforations, often with yellow margins. Rotala rotundifolia shows it within a week of onset; slower-growing species like Anubias may take a month. New growth remains normal because plants translocate potassium from old leaves to support emerging tissue. Cross-reference the potassium deficiency aquarium plants piece for visual identification.

Over-Dosing Concerns

Plants tolerate potassium up to about 50 ppm sustained without problems. Above that, K starts to antagonise calcium and magnesium uptake, manifesting as twisted new growth or magnesium-deficiency symptoms even when Mg is present. In soft Singapore tap water with low baseline calcium, this antagonism shows earlier than in hard-water systems. Keep K below 40 ppm in soft-water shrimp tanks and you avoid the issue entirely.

Sourcing K2SO4 in Singapore

Horticultural K2SO4 is sold at agricultural supply shops in Sungei Kadut and on Shopee at $8 to $12 per kilogram. Aquarium-branded pouches at C328 cost $15 to $20 for 250 grams. Both are the same compound; the bulk horticultural source is dramatically better value. Avoid mixed potassium fertiliser blends; pure K2SO4 keeps your dosing maths clean.

Dosing Schedule for a 60 Litre Tank

For an EI-style 60 litre tank already dosing KNO3 and KH2PO4 three times weekly, add 0.3 grams of K2SO4 (1.5 ml of 200 g/L stock) on Tuesday, Thursday and Saturday. That layers onto the macro days without disturbing the schedule and delivers approximately 4 ppm K per dose, 12 ppm weekly. Adjust upwards if pinhole damage persists, downwards if calcium-deficiency symptoms appear. Pair the schedule with the structure in our aquarium ei dosing complete guide.

Interaction With Calcium and Magnesium

High potassium suppresses calcium and magnesium uptake by competing for root and leaf transport channels. In soft-water Singapore tanks where Ca is already low (typically 10 to 20 ppm from PUB tap), heavy K dosing can trigger calcium deficiency before plant K targets are met. Monitor for new-growth distortion alongside K dosing increases. The aquarium calcium magnesium guide covers the balance.

When Standalone K Is Not the Answer

If pinhole damage persists after two weeks of correct K2SO4 dosing, suspect CO2 limitation rather than nutrient deficiency. Plants cannot use potassium without adequate carbon, and pinhole symptoms in carbon-starved tanks mimic K deficiency closely. Verify CO2 at 25 to 30 ppm via drop checker and pH-kH calculation before adding more salts. Our aquarium co2 measurement guide drop checker ph walks through the verification.

Related Reading

• Aquarium Potassium Dosing Guide
• Potassium Deficiency Aquarium Plants
• Aquarium EI Dosing Complete Guide
• Aquarium Dry Fertiliser Mixing Guide
• Lean Dosing Method Planted Tank