Plant deficiency guide
Phosphorus Deficiency
Also called low phosphate, phosphate deficiency
Older leaves darken with purple or bronze tints, and green spot algae creeps onto the glass and slow leaves.
TL;DR, Phosphorus Deficiency
Phosphorus is mobile, so shortages show on older leaves. They dull, darken and can pick up purple or reddish-bronze tones, and growth slows. The reliable tank-wide tell is green spot algae appearing on the glass and on old, slow leaves like anubias, which is your cue that phosphate has bottomed out. Add phosphate, usually through an all-in-one fertiliser, and the spot algae eases as the plants get fed. Lightly stocked tanks and tanks full of fast plants run out of phosphate most often.
What you see
Older leaves lose their fresh green and take on a dark, dull, sometimes purple or bronze cast. Growth slows and leaves can look small. On the glass and on old, slow-growing leaves you get hard little green dots, green spot algae, which is the most reliable sign that phosphate is limiting.
What causes it
- Phosphate has dropped near zero, common in light-stock tanks with no dosing
- Heavy uptake from a large, fast-growing plant mass
- Some substrates and media bind phosphate out of the water
- A new tank still settling into a nutrient rhythm
How to fix it
- Test phosphate if you have a kit. A working range is around 1 ppm.
- Dose phosphate through an all-in-one, or a small dose of monopotassium phosphate.
- Scrape the green spot algae now, since it will not lift once it hardens.
- Let nerite snails graze what is left on the glass and hardscape.
- Hold a steady low phosphate level rather than letting it swing to zero.
Keep it from coming back
Dose phosphate alongside nitrogen and potassium instead of letting it hit zero. Watch the glass, because returning green spot algae is your earliest signal that phosphate is running short.
Easy to confuse with
Green spot algae also appears under strong light on slow leaves, so treat low phosphate and light together. The purple leaf tint can look like a lean-nutrient red, but here it comes with dulling and slow growth, not vigour.
Plants that show it first
Compare against a species you keep. These plants flag phosphorus trouble early or clearly:
- Anubias Barteri — Slow leaves collect green spot algae first when phosphate is low, an easy visual cue.
- Anubias Barteri Nana — Same story on a smaller leaf, the spots show clearly on old growth.
- Java Fern — Old fronds pick up green spot algae when phosphate bottoms out.
- Amazon Sword — Large old leaves dull and darken, and spot algae settles on them.
Frequently asked questions
Direct answers to the questions search engines and AI assistants surface most often about this species.
Does phosphate cause algae?
It is mostly a myth for planted tanks. Low phosphate is a common cause of green spot algae, not high phosphate. Starving phosphate to fight algae usually backfires.
What phosphate level should I aim for?
Around 1 ppm is a comfortable working level for a growing planted tank. Steady matters more than exact.
Green spot algae keeps returning on my glass, why?
It is the classic sign of low phosphate, often combined with strong light on slow surfaces. Raise phosphate and it eases.
Do I need a separate phosphate dose?
Not if your all-in-one already includes it. Only add extra phosphate if testing shows it bottoming out.