Magnesium for Aquarium Plants: Deficiency Signs and How to Dose

Magnesium sits at the centre of every chlorophyll molecule, so plants can’t make the green that powers growth without it. Most hard tap water has plenty. Soft water, rainwater and RO water often don’t. When magnesium runs short, plants pull it out of their old leaves to feed new ones, and that’s the pattern you’ll see.

What magnesium does for aquarium plants

Magnesium has two big jobs. It forms the core of chlorophyll, the pigment that turns light into energy. It also switches on many of the enzymes plants use to build sugars and proteins. A plant short of magnesium can’t use the light you give it, however good your lighting is.

Plants need far more magnesium than trace elements, which is why it’s treated as a macro nutrient. In a complete aquarium plant food it sits in the macro bottle alongside nitrogen, phosphate and potassium.

Signs of magnesium deficiency

Magnesium is mobile inside the plant. When it runs low, the plant moves it from older leaves to newer ones. So the signs show up on the older, lower leaves first:

• Yellowing between the veins while the veins stay green, a striped or netted look
• Older leaves turning pale from the edges inwards
• Leaves dropping from the bottom of stem plants
• Healthy-looking new growth at the top, at least at first

If the pale growth is at the top of the plant instead, magnesium is less likely. New leaves turning pale with green veins usually points to iron deficiency.

Who runs short of magnesium?

Most UK hard water areas supply plenty of magnesium straight from the tap. Shortages are most common when:

• You use RO, rainwater or very soft tap water
• You remineralise for shrimp with a calcium-heavy product
• You have a heavily planted, high-light tank with injected CO₂, where plants use nutrients fast
• Your water report shows a high calcium to magnesium ratio

Your water company publishes a free water quality report by postcode. Check the magnesium figure. Very low numbers are a good hint you need to add some.

How to dose magnesium

The simplest fix is magnesium sulphate (MgSO₄), the same compound sold as Epsom salt but in a purer grade. In the PPS-Pro system it goes into the macro bottle, so magnesium is dosed every day with the rest of your plant food.

If you already dose a complete fertiliser and still see signs of magnesium deficiency, your water is the likely cause. Add a little extra magnesium sulphate to your macro bottle, or dose it separately after water changes. Make small changes and watch new growth over two to three weeks before adding more.

Is magnesium safe for shrimp and fish?

Yes, at sensible doses. Magnesium is part of general hardness (GH), which shrimp need for healthy moults. Adding magnesium sulphate raises GH slightly, so keep an eye on your GH test if you keep sensitive species such as Caridina. See our guide on whether aquarium fertiliser is safe for shrimp.

Quick answers

What does magnesium deficiency look like in aquarium plants?

Older, lower leaves turn yellow between the veins while the veins stay green. New growth usually looks fine at first.

Can I use Epsom salt as a magnesium fertiliser in my aquarium?

Epsom salt is magnesium sulphate, but garden or bath grades can contain additives. Use a pure grade made for aquariums or plants.

How do I know if my tap water has enough magnesium?

Check your water company’s quality report by postcode. Soft water areas and RO water are the most likely to be low.

Does magnesium raise GH?

Yes. Magnesium and calcium together make up general hardness, so adding magnesium sulphate raises GH a little.

Products mentioned in this guide