Dosing & Chemistry
Potassium (K) Calculator
Work out the grams of potassium sulphate (K2SO4) needed to hit a target potassium PPM in your planted aquarium.
Potassium (K) Calculator
20 ppm suits most planted tanks. 10-30 ppm is a safe range.
From a test kit, or 0 if unknown.
K2SO4 to add
3.34g
- Approx. teaspoons
- 0.61 tsp
- ppm to raise
- 15 ppm
~5.5 g per level teaspoon of K2SO4.
How the potassium dose is worked out
Potassium sulphate (K2SO4) is 44.87% potassium by mass. So to add a given ppm of K to your tank water you need rather more K2SO4 than the ppm figure suggests:
grams K2SO4 = (ppm deficit x tank volume L) / 1000 / 0.4487
Worked example
A 100 L tank, currently 5 ppm K, target 20 ppm: deficit is 15 ppm, so 15 x 100 / 1000 / 0.4487 = 3.34 g of K2SO4. That's about 0.6 of a level teaspoon. Dissolve in tank water first, then dose.
Practical tips
Dry ferts are far cheaper than bottled liquids (a single 500 g bag of K2SO4 costs under GBP 10 and will dose an average tank for years), but they require a digital scale accurate to 0.1 g for small tanks. If you're dosing under 0.5 g, make up a liquid stock solution - 50 g K2SO4 in 500 mL RO water - and dose that instead. 10 mL of that solution delivers 1 g of K2SO4.
Frequently asked questions
Why do my aquarium plants need extra potassium?
Potassium is one of the three core plant macronutrients (N, P, K). Fish food and tap water rarely supply enough, especially in a soft-water UK tank. Symptoms of low potassium include pinholes and yellow patches in older leaves, and slow or stunted growth in Rotala and stem plants.
What ppm of potassium should I target?
Most planted-tank guides (EI, PPS-Pro, Tropica, ADA) target 10-30 ppm of K. 20 ppm is a common sweet spot - enough for fast stems and carpets, without pushing other nutrients out of balance. Very high K (50+ ppm) has been linked to calcium uptake issues in soft water, so don't overshoot without reason.
Is K2SO4 (potassium sulphate) safe for fish and shrimp?
At normal planted-tank doses, yes - it's one of the most fish- and shrimp-safe dry ferts available. The sulphate anion has no significant effect at these levels. Always dissolve in RO or tank water before adding to the tank rather than sprinkling the dry powder directly.
How is this different from dosing an all-in-one fertiliser?
Bottled all-in-ones (TNC Complete, APT Complete, EI Liquid) already include potassium, so a straight K dose is only needed when you're running into a K-specific deficiency or dosing macros separately. This calculator is most useful for DIY dosers using dry salts.
Can I use potassium nitrate (KNO3) instead?
Yes - KNO3 is 38.7% potassium by mass, very similar to K2SO4. If you're already dosing KNO3 for nitrate, check that your overall K number comes out to your target before adding extra K2SO4, or you can easily overshoot.
Related calculators
Dosing
Scale any liquid aquarium product (ferts, dechlorinator, treatments) to your exact tank volume.
Ammonia
Work out the millilitres of household ammonia (or similar) to dose a fishless cycle to a target ppm.
Lighting
Recommended lighting power in watts or lumens per litre for low, medium and high tech planted tanks.