Potassium for Aquarium Plants: Deficiency Signs and Supplements
Pinholes, yellow edges and leaves that fall apart are classic potassium problems. Here’s why planted tanks run short and the easiest way to fix it.
Potassium is the nutrient planted tanks run short of most often. Fish food supplies some nitrogen and phosphate, but very little potassium, and tap water rarely has much. So unless you add it, plants in a busy tank soon run low. The good news is it’s one of the easiest and safest nutrients to dose.
What potassium does
Potassium controls how water and sugars move through the plant. It opens and closes the pores on leaves, helps build strong cell walls and switches on enzymes for growth. Plants use a lot of it, which is why it’s one of the three main macro nutrients alongside nitrogen and phosphate.
Signs of potassium deficiency
Like magnesium, potassium is mobile, so the signs appear on older leaves first:
- Small pinholes that grow into larger holes
- Yellow or brown edges and tips on older leaves
- Leaves that go thin and see-through before falling apart
- Slow growth even with good light and CO₂
Pinholes are the giveaway. If you see them on java fern, anubias, swords or hygrophila, potassium is the first thing to check. Our guide to pinholes and brown patches shows what to look for.
Potassium nitrate vs potassium sulphate
There are two common ways to add potassium. Which one you need depends on your nitrate level.
| Compound | Adds | Best for |
|---|---|---|
| Potassium nitrate (KNO₃) | Potassium + nitrogen | Most tanks. Feeds two nutrients at once |
| Potassium sulphate (K₂SO₄) | Potassium only | Tanks where nitrate is already high |
| Monopotassium phosphate (KH₂PO₄) | Potassium + phosphate | Small amounts as your phosphate source |
The PPS-Pro macro bottle uses all three. Potassium nitrate supplies nitrogen, potassium sulphate tops up potassium without pushing nitrate up, and monopotassium phosphate adds phosphate. That balance is why PPS-Pro rarely leaves plants short of potassium.
How much potassium to dose
With a balanced daily fertiliser you don’t need to work out potassium separately. Dose 1ml per 10 litres of the macro solution every day and adjust to your light level. Our dosing guide covers low, medium and high-light tanks.
If you’ve been dosing and still see pinholes on older leaves, add a little extra potassium sulphate to your macro bottle. Potassium is very forgiving. Small overdoses don’t harm fish or shrimp, and plants simply take up what they need.
In tanks with lots of fish, nitrate and phosphate build up from food and waste, but potassium doesn’t. That’s why tanks with plenty of fish can still have potassium-starved plants.
Is potassium safe for shrimp?
Yes. Potassium at the levels used for plants is safe for shrimp, snails and fish. Our compounds are copper-free and made for planted tanks with invertebrates.
Quick answers
What is the best potassium supplement for an aquarium?
Potassium sulphate is the best stand-alone potassium supplement because it adds potassium without adding nitrate. Potassium nitrate is best if your plants also need nitrogen.
What does potassium deficiency look like in aquarium plants?
Pinholes in older leaves, yellow or brown edges and leaves that turn thin before breaking up.
Can you overdose potassium in an aquarium?
It is hard to overdose at plant-food levels. Small excesses are harmless to fish and shrimp.
Does fish food provide potassium?
Very little. Fish food mainly adds nitrogen and phosphate, so potassium is usually the first nutrient to run short.
Products mentioned in this guide
Adds potassium without adding nitrate. The one to reach for when you want more potassium but nitrate is already high.
Nitrogen + potassiumThe main source of nitrogen and potassium in your macro bottle. Usually the first compound to run low.
Best valueEverything to start dosing on day one: pre-measured macro and micro compounds, two 500ml bottles, two dosing syringes and a full dosing guide.
Copper-free aquarium plant food.
Dose on day one.
Pre-measured compounds, bottles, syringes and a full dosing guide. Shrimp safe, made in the UK, free UK delivery on every order.
Everything you need to get the most from your planted aquarium.
