Potassium Dosing in Planted Aquariums: The Forgotten Macro
Potassium is one of the three essential macronutrients for aquarium plants, alongside nitrogen and phosphorus. Yet it is often the most overlooked, as fish waste and food provide almost no potassium to the water. Without deliberate supplementation, planted tanks frequently suffer from potassium deficiency — and many hobbyists blame the symptoms on other issues. This potassium dosing planted aquarium guide from Gensou Aquascaping at 5 Everton Park explains why potassium matters and how to get it right.
Why Plants Need Potassium
Potassium (K) plays a critical role in plant physiology. It regulates the opening and closing of stomata (the gas exchange pores), activates enzymes involved in photosynthesis, assists in protein synthesis, and helps transport nutrients within the plant. Unlike nitrogen and phosphorus, potassium does not become part of the plant’s structure — it remains dissolved in cell fluid, meaning plants need a continuous supply.
Signs of Potassium Deficiency
The classic symptom is small pinholes in older leaves, often starting with mid-level and lower leaves. These holes have yellow or brown margins and gradually expand. Leaf edges may turn yellow or brown and curl. Growth slows even when nitrogen, phosphorus, CO2 and light are adequate. Potassium deficiency is commonly misidentified as a phosphorus or iron issue. If pinholes appear on older leaves while new growth looks healthy, potassium is the likely culprit.
Potassium Sources for Aquariums
Potassium sulphate (K₂SO₄): The most common dry salt for dosing potassium. It adds only potassium and sulphate, both of which plants use. Inexpensive and widely available from fertiliser suppliers.
Monopotassium phosphate (KH₂PO₄): Supplies both potassium and phosphate. Useful when both nutrients are needed, as in EI dosing.
Potassium nitrate (KNO₃): Supplies potassium and nitrogen together. Commonly used in EI and PPS-Pro dosing methods.
Commercial liquid fertilisers: Many all-in-one or dedicated potassium supplements are available in Singapore. Seachem Flourish Potassium and APT Complete are popular options. These are convenient but more expensive per dose than dry salts.
How Much to Dose
For a moderately planted tank, target potassium levels of 10–30 ppm. In an EI dosing regimen, potassium is dosed at roughly 20 ppm per week through a combination of KNO₃ and K₂SO₄. For lean dosing methods, aim for 5–10 ppm per week and adjust based on plant response. There is no standard test kit for aquarium potassium, so dosing is based on calculated amounts rather than testing. Use an online aquarium fertiliser calculator to determine the right amount for your tank volume.
Dosing Schedule
Divide your weekly potassium dose across two to three dosing days, alternating with micronutrient days. For example, dose macronutrients (including potassium) on Monday, Wednesday and Friday, and micronutrients on Tuesday, Thursday and Saturday. Perform a 50 per cent water change on Sunday to reset nutrient levels. This schedule ensures consistent availability without excessive accumulation.
Can You Overdose Potassium?
Potassium is one of the safest nutrients to dose. Fish and invertebrates tolerate levels well above what plants need. In extreme excess (above 50 ppm), some hobbyists report reduced growth, but this rarely occurs in normal dosing routines. The practical advice is not to worry about overdosing — potassium deficiency is far more common and more damaging than excess.
Potassium and Singapore Tap Water
Singapore’s tap water contains negligible potassium — typically less than 2 ppm. This means every planted tank in Singapore needs supplemental potassium. If you use RO water and remineralise with GH boosters, check whether your product includes potassium — some do (like Salty Shrimp GH+) while others focus on calcium and magnesium only. Supplementary dosing is almost always needed regardless of water source.
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