Most deficiency charts do more harm than good, because they let you look up a symptom and start dosing without ever checking whether a nutrient was the problem. In a CO2 injected aquarium, the majority of symptoms that look like deficiency turn out to be something else. This guide starts by ruling those out, then works through what a real deficiency actually looks like.
Before you diagnose anything
Five things cause symptoms that look exactly like nutrient deficiency and are far more common. Work through these first, because adding fertiliser to any of them makes the tank worse rather than better.
CO2. In an injected tank this is the single most likely cause. Insufficient CO2, or CO2 that is present but not reaching part of the tank, produces stunted tips, small leaves, pale growth and holes. The giveaway is that it usually affects plants in one area rather than the whole tank.
Flow. Nutrients arrive by water movement. A plant sitting in a dead spot can starve in a tank with perfect water parameters. If symptoms cluster in one corner, or on the plants furthest from the outflow, flow is your first suspect.
Light. Too little gives leggy, pale, undersized growth. Too much without matching CO2 gives damaged leaves and algae. Neither is fixed by fertiliser.
Water chemistry. High KH holds pH high, and at high pH iron and manganese become progressively unavailable no matter how much you dose. If a deficiency persists in a tank where you are dosing properly, see the KH section further down.
Transition and age. A plant that has just gone in will often melt or drop its emersed leaves regardless of what you dose. That is normal. Equally, the oldest leaf on a bucephalandra is meant to die eventually. One deteriorating old leaf is not a deficiency.
The test is simple. Is it affecting new growth, across the whole tank, consistently? If yes, a deficiency is plausible. If it is one plant, one corner, or only the oldest leaves on a plant that is otherwise growing well, look at the five above first.
The one question that halves the work
Plants can move some nutrients around internally and not others. That single fact tells you where to look.
Where a symptom appears tells you as much as what it looks like. Check whether it is new growth or old before anything else.
Mobile nutrients can be pulled out of old leaves and sent to new growth. When supply runs short, the plant cannibalises its old leaves to keep the new ones going, so symptoms appear on old, lower leaves first.
Immobile nutrients are locked into the tissue they were built into. The plant cannot recover them, so shortage shows up in whatever is being built right now, on new growth, while old leaves stay perfectly fine.
Potassium
The most common deficiency in a planted aquarium, and the cause behind most reports of holes in leaves.
Potassium deficiency on an older leaf. Holes typically begin as pale spots that darken, die and then perforate, scattered across the leaf rather than concentrated at one edge.
- What it looks like
- Small holes in older leaves, often starting as pale or yellow pinprick spots that darken, die and perforate. Leaf margins may yellow or brown. Holes are scattered across the leaf rather than concentrated at the edge.
- Where it shows
- Older leaves first, because potassium is mobile.
- Which plants flag it first
- Bucephalandra and Hygrophila pinnatifida are heavy potassium feeders and will show it earliest and most reliably. If either is developing pinholes while the rest of the tank looks fine, potassium is the first thing to check. Anubias and larger swords show it too, just later.
- Why it happens
- Unlike nitrogen and phosphate, potassium is not supplied by fish waste in any meaningful quantity. So a heavily stocked tank can still run short, and a lightly dosed or unfertilised tank will exhaust its potassium long before its nitrogen.
- Rule out first
- CO2 deficiency also produces holes, as does simple old age in a bucephalandra leaf. The distinction is pattern. Potassium shows across multiple older leaves on multiple plants. CO2 shows on new growth and in particular parts of the tank.
All four AquaLabs liquid fertilisers supply potassium, including Easy Grow Lite, which adds potassium, magnesium, iron and traces without nitrate or phosphate. That combination suits the tank most likely to hit potassium deficiency: plenty of fish supplying nitrogen, not much else being added.
Nitrogen
- What it looks like
- Uniform yellowing of the whole leaf, veins included, starting with the oldest leaves. Growth slows. In some species older leaves go translucent or fall away. Reds may intensify, which is why nitrogen limitation is used deliberately in some tanks.
- Where it shows
- Older leaves first.
- Why it happens
- Most often in a well planted tank with a low fish load, or one running lean without a rich substrate underneath. Fish waste supplies nitrogen, so heavily stocked tanks rarely run short.
- One important caution
- A nitrate reading of zero in a well planted CO2 tank often means healthy uptake rather than shortage. Judge by the plants. If growth is good and colour is strong, a low reading is fine.
Iron and magnesium
These two produce the same symptom in different places, and confusing them is the most common misdiagnosis in the hobby. People dose iron for months at what is actually a magnesium problem.
Identical symptom, opposite position on the plant. Iron is immobile so it shows on new growth. Magnesium is mobile so the plant strips it from old leaves first. Dosing iron at a magnesium problem will not help.
- What it looks like
- Interveinal chlorosis on new growth. Tissue between the veins turns yellow or almost white while the veins stay green, giving a net or skeleton pattern. In severe cases new leaves emerge pale and stay small. Red plants lose colour and go pale green or yellow.
- Where it shows
- New growth only, because iron is immobile. Older leaves staying perfectly green is the clearest signal you will get.
- Why it happens
- Iron is difficult to keep available in aquarium water. It precipitates out readily, particularly at higher pH, which is why fertilisers use chelated forms. High pH and high KH shorten how long dosed iron stays available.
- What it looks like
- Interveinal chlorosis, the same pattern as iron.
- Where it shows
- Old leaves, because magnesium is mobile. This is the whole distinction.
- Why it happens
- Soft water. If you are on very soft tap water, or using RO or rainwater without adequate remineralisation, magnesium is a genuine possibility. In hard water areas it is unlikely, because tap water supplies plenty.
- Where this matters in Australia
- Melbourne and much of coastal Victoria run very soft water, as do parts of Tasmania and southeast Queensland. Adelaide and Perth are considerably harder. Supply varies between zones even within a city, so check your own GH rather than assuming from your postcode.
Test your general hardness before assuming anything. The AquaLabs GH Test Kit measures calcium and magnesium levels directly, and takes the guesswork out of whether soft water is your problem.
If GH is low, AquaLabs GH Plus raises it with calcium and magnesium. Potassium and chloride are deliberately excluded from the formulation, so it will not interfere with the NPK ratios your fertiliser is providing or stunt growth through excess chloride.
When the problem is your KH
There is a category of deficiency that no amount of extra dosing will fix, and it catches out people trying to grow the most rewarding plants in the hobby.
If you are struggling with Hygrophila chai, Eriocaulon, Tonina or Syngonanthus species, and everything else in the tank looks healthy, your carbonate hardness is the first thing to check. These plants evolved in blackwater habitats with almost no dissolved carbonate and a pH in the low fives. Put them in hard, alkaline water and they will decline regardless of how well you dose.
Why carbonates get in the way
KH measures bicarbonate and carbonate. Its job is to buffer, which means resisting change in pH. The higher your KH, the more stubbornly your pH sits where it is, and in most Australian tap water that means sitting high.
Bicarbonate use separates the tough plants from the sensitive ones. Many aquarium plants can strip carbon out of bicarbonate when dissolved CO2 runs short. Vallisneria, elodea and many swords do it readily, and you can see the evidence as the white calcium crust they leave on their leaves. The soft water species cannot. In hard water with limited CO2, the tough plants keep growing while the sensitive ones sit there carbon starved and are gradually outcompeted.
How much of the difficulty with these species is nutrient availability, and how much is a direct physiological intolerance of hard water, is not fully settled. Both are almost certainly at work. What is not in doubt is the practical result: lower the KH and they grow.
What to aim for
Most planted aquariums are perfectly happy at 3 to 6 dKH, and there is no reason to chase a lower number if your plants are growing well.
For the sensitive species above, aim considerably lower. 0 to 2 dKH suits most, and Tonina and Syngonanthus generally want less than 1. That is low enough for pH to sit in the low sixes with CO2 running, which is where iron stays available and where these plants evolved.
There is a genuine trade-off in going that low, and it is worth stating plainly. KH is your buffer. Take it away and pH becomes far more volatile, so a CO2 solenoid failure or a heavy overnight drop can swing pH much further than it would in a buffered tank. Very low KH is a specialist choice for a specific plant list, not a general improvement.
AquaLabs KH Minus permanently neutralises carbonate hardness, converting excess alkalinity into CO2 and water. Add 4ml per 30 litres to reduce KH by 1 dKH.
Reduce by no more than 2 dKH in any 24 hour period, and remember that lowering KH will lower pH along with it. Add KH Minus to water rather than water to KH Minus, and never dose directly onto plants or livestock.
Test before and after with the AquaLabs KH Test Kit.
Check your fish and shrimp first
Water chemistry is not just a plant decision. Soft water suits caridina shrimp, most tetras, rasboras and other blackwater fish, and they will often do better at lower KH too. Livebearers, most cichlids and neocaridina shrimp prefer harder water and should not be dragged down to 1 dKH to suit a plant.
If your stocking wants hard water and your plant list wants soft, that is a conflict to resolve before you start dosing anything, not after.
Phosphate
- What it looks like
- Stunted growth, older leaves darkening or taking on purple or bronze tones, sometimes with dark spotting. Less distinctive than the others and easy to confuse. Green spot algae will often pop up when Phosphate is low also.
- Where it shows
- Older leaves first.
- Why it happens
- Rare in a stocked aquarium, because fish food and waste supply phosphate continuously. Most likely in a heavily planted tank with few fish, or one being deliberately dosed lean.
Calcium
- What it looks like
- Deformed, twisted or stunted new growth. New leaves may emerge crumpled or fail to open properly. Growing tips can die back.
- Where it shows
- New growth only. Calcium is immobile and moves with water uptake.
- Why it happens
- Very soft water without remineralisation. Same regional pattern as magnesium, and the same fix.
Reading the symptom, not the chart
Two questions get you most of the way there.
Old growth or new growth? Then what is the pattern?
- Old leaves, uniform yellowing including the veins: nitrogen
- Old leaves, yellow between green veins: magnesium
- Old leaves, scattered holes with yellow halos: potassium
- Old leaves, darkening or bronzing with stunted growth: phosphate
- New growth, yellow between green veins: iron
- New growth, deformed or twisted: calcium
What to actually do
Do not chase a single nutrient. Deficiencies rarely arrive alone, because they usually mean the whole dosing regime is short rather than one element being absent. Someone dosing iron at what is a general shortage will chase the symptom for months without fixing it.
The better move is to correct the whole regime, then reassess after a few weeks of consistent dosing. If a specific deficiency persists once everything else is right, then treat it individually.
Start by making sure the formulation matches your aquarium. A tank with plenty of fish and few plants needs something very different to a densely planted CO2 scape, and using the wrong one is a common cause of persistent deficiency.
Give it time. Damaged leaves do not recover. Potassium holes do not close up and chlorotic leaves do not re-green. You are looking for healthy new growth, which takes two to four weeks to be obvious in a low tech tank and one to two in a CO2 tank.
Change one thing. If you adjust fertiliser, light and CO2 in the same week, you will learn nothing from the result.
Consistency beats precision. Plants adjust to what is reliably available. A moderate dose every day beats a heavy dose whenever you remember.
Common mistakes
Worth avoiding
- Diagnosing from a photo without first checking whether the affected leaf is new growth or old.
- Dosing iron for interveinal chlorosis on old leaves. That is magnesium, and iron will not fix it.
- Dosing more iron into a high pH tank. If KH is holding pH above 7, most of what you add will precipitate before the plants reach it.
- Treating one dying old leaf as a deficiency. Old leaves are supposed to die.
- Adding fertiliser at a CO2 or flow problem. The most common mistake of all in an injected tank.
- Expecting damaged leaves to recover. Judge by what grows next, not by what is already marked.
- Dosing several corrective products at once, which guarantees you will not know what worked.
The short version
- Rule out CO2, flow, light, water chemistry and transition melt before diagnosing anything
- Old growth means a mobile nutrient: nitrogen, phosphorus, potassium or magnesium
- New growth means an immobile one: iron or calcium
- Holes in older leaves, especially on buce and pinnatifida, means potassium
- Uniform yellowing of old leaves means nitrogen
- Interveinal chlorosis on new growth is iron, on old growth is magnesium
- Deformed new growth means calcium, usually in very soft water
- Persistent iron deficiency despite consistent dosing usually means pH is too high, which means KH is too high
- Soft water species like chai, eriocaulon and tonina need KH around 1 to 2 dKH or lower
- Test GH and KH before assuming anything about hardness
- Fix the whole dosing regime rather than chasing one element
- Judge by new growth, and give it a few weeks
Deficiencies are worth learning to read, because the alternative is dosing at symptoms and hoping. But the first question is always whether a nutrient was ever the problem, and in a CO2 injected aquarium it very often was not.