You can dose a fertiliser perfectly and still see deficiency, because dosing a nutrient is not the same as making it available. pH decides how much of what you add stays in a form plants can actually take up, and for the trace metals the difference between good water and bad water is not subtle. This is why chelates exist, and why which chelate a fertiliser uses matters.
Available is not the same as present
A nutrient is only useful to a plant if it is dissolved in a form the plant can absorb. Iron dosed into your tank that has reacted with oxygen and dropped to the substrate as rust is still iron. It is simply no longer available.
This is the gap that catches people out. The bottle went in, the dose was correct, the test kit might even show something, and the plants are still short. What changed between the bottle and the leaf is chemistry, and pH is what drives it.
How availability changes with pH
Simplified, and deliberately so. The macronutrients hold up across the whole range. The trace metals are the ones that fall away, and they fall away exactly where most tap water sits.
Two things stand out.
The macronutrients barely care. Nitrogen, potassium, calcium and magnesium remain available across the whole range you would ever run an aquarium at. If you are dosing nitrate and potassium, pH is not what is stopping your plants using them.
The trace metals care enormously. Iron, manganese, copper and zinc all become progressively unavailable as pH rises, and the drop is steep rather than gradual. This is why iron deficiency is so much more common than potassium deficiency in hard water, and why the same fertiliser can work beautifully in one tank and appear to do nothing in another.
Why metals fall out of solution
Iron in your fertiliser is in the ferrous form, Fe2+, which plants take up readily. In oxygenated water it oxidises to ferric iron, Fe3+, and ferric iron reacts with hydroxide ions to form iron hydroxide, which is insoluble.
Hydroxide concentration is what pH measures. Higher pH means more hydroxide, which means faster precipitation. The iron does not disappear. It becomes a fine rust that settles into the substrate where no plant can reach it, and over time it is part of what leaves that brown film on hardscape and glass.
The other trace metals have the same problem through slightly different routes.
- Manganese behaves much like iron. It oxidises and precipitates as manganese oxide, and like iron it does so faster as pH climbs.
- Copper precipitates as copper hydroxide, and in carbonate-rich water as copper carbonate. Hard alkaline water gives it both routes at once.
- Zinc precipitates as zinc hydroxide, but its bigger problem is adsorption. Zinc readily binds to surfaces such as substrate, filter media and organic matter, and once bound it is effectively gone.
None of these are exotic reactions. They are simply what these metals do in water containing oxygen and carbonates, which is every aquarium.
Phosphate and metals precipitate each other
There is a second problem that has nothing to do with pH, and it explains something about how fertilisers are built.
Iron phosphate and zinc phosphate are both poorly soluble. Put unprotected iron and phosphate in the same solution and they will find each other, forming a precipitate that removes both from availability at once.
This is why concentrated fertiliser programmes traditionally split macronutrients and micronutrients into separate bottles, dosed on alternate days. Keeping the phosphate away from the iron was the only way to stop them reacting.
An all-in-one fertiliser carries both in the same bottle, which means the iron has to be protected from the phosphate sitting next to it for the entire shelf life of the product, and then again once it is in your tank.
Chelation is what makes it work
A chelate is a molecule that wraps around a metal ion and holds onto it. The word comes from the Greek for claw, which is a fair description. While the metal is held, it cannot react with hydroxide, it cannot react with phosphate, and it cannot adsorb onto surfaces. It stays dissolved and it stays available.
The plant then releases the metal from the chelate at the leaf or root surface when it needs it, so chelation protects the nutrient in transit without blocking uptake at the destination.
Worth stating plainly: chelation is not a refinement on a fertiliser, it is the thing that allows a complete fertiliser to exist at all. Without it you would be back to separate macro and micro bottles on alternate days, and even then your iron would precipitate out of the water column within hours in anything but soft acidic water.
Every serious aquarium fertiliser uses chelated trace metals. What separates them is which chelate, because chelates are not equal and each has a pH range beyond which it stops holding on.
The three iron chelates, and their limits
Each chelate holds iron reliably up to a point, then progressively releases it to precipitate out. Beyond its range, a chelate is doing very little.
There is a trade-off running through all of this, and it is the reason no single chelate is the right answer. Stability and availability pull in opposite directions. The more tightly a chelate grips the metal, the better it survives high pH, and the harder the plant has to work to prise it loose at the leaf surface.
EDTA is the most common and the cheapest. It holds iron the least tightly of the three, which means plants take it up most readily, and it is also the first to give up as pH rises. It works well in acidic water and becomes increasingly ineffective above roughly pH 6.5. In a soft water CO2 tank running at pH 6.2, EDTA iron is entirely adequate and arguably ideal. In hard alkaline tap water at pH 7.8, a large share of it will precipitate before the plants get to it.
DTPA holds on considerably further, to around pH 7.5. Uptake is slightly less immediate than EDTA, but it covers the range a great deal of Australian tap water actually sits in. It costs more.
EDDHA is stronger again and remains stable well above pH 9. That stability comes at the cost of the most difficult release of the three.
Why AquaLabs uses EDTA and DTPA together
Because of that trade-off, using a single chelate means accepting a compromise at one end or the other. EDTA alone leaves harder water customers short. DTPA alone gives up some of the easy uptake that EDTA delivers in soft water.
Combining the two covers both. EDTA supplies iron in the most readily absorbed form for tanks at the acidic end, and DTPA continues holding iron in solution as pH climbs into the range where EDTA has started to let go. Between them they span the water most aquarists are actually running.
Why not EDDHA
On paper EDDHA looks appealing, since it is by far the most stable of the three. We do not use it, for three reasons.
Uptake. Iron availability to the plant runs in the opposite direction to stability, and EDDHA sits at the difficult end. It holds iron beautifully in solution while making it harder for the plant to actually take up, which defeats much of the purpose of dosing it.
It does not blend well. EDDHA does not hold in a stable mixed solution alongside other chelated metals, and a fertiliser that separates over time is not delivering a consistent dose from the first pump to the last.
It stains. EDDHA iron is intensely coloured and tints aquarium water a distinct red or wine tone at quite modest doses. In a display aquarium that is not a minor cosmetic issue.
EDDHA has a genuine place in agriculture and hydroponics, where water is often much harder, dosing is continuous and clarity does not matter. In a display aquarium the case for it is weaker.
All AquaLabs liquid fertilisers supply iron as a combination of EDTA and DTPA chelates, so availability holds across a wider pH range than EDTA alone while keeping iron in a form plants absorb readily. Copper, manganese and zinc are chelated with EDTA.
Easy Grow, Easy Grow Lite, Lean Grow and Max Grow all use the same chelation approach.
What this means for your tank
| Your pH | What is happening |
|---|---|
| Below 6.5 | Trace metals stay available easily. Iron chelation is not a limiting factor. Common in CO2 injected soft water tanks. |
| 6.5 to 7.5 | The range most planted tanks sit in. EDTA alone starts losing ground here, and DTPA is doing real work. |
| Above 7.5 | Trace metal availability drops considerably. Expect iron deficiency symptoms on new growth even with correct dosing, and consider whether lowering KH is the better fix than dosing more. |
Worth noting that pH in a CO2 injected tank moves through the day. It falls as CO2 comes on and rises again overnight. What matters for availability is roughly the average rather than any single reading, so test at the same time each day if you are tracking it.
pH is set by your KH
You cannot adjust pH directly in any lasting way, and trying to is a well known route to trouble. Products that push pH down without addressing carbonate hardness get buffered straight back, often with a swing that stresses livestock on the way.
KH is the buffer. Lower the carbonate hardness and pH follows. Raise it and pH climbs. If your pH is sitting too high for good trace metal availability, KH is the lever, not pH.
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, and never reduce by more than 2 dKH in 24 hours.
What to actually do
Test your pH and KH before changing anything. Most people diagnosing a persistent trace deficiency have never measured either.
Do not simply dose more iron. If pH is the problem, more iron means more precipitate, not more uptake. You will spend money and stain your substrate without fixing anything. All AquaLabs fertilisers contain ample iron, without the need to dose more seperately.
Consider whether the pH is worth changing at all. Plenty of aquariums run happily at 7.5 with slightly reduced trace availability and no visible problem. This matters when you are seeing symptoms, not as a target to chase.
Match your plants to your water where you can. Fighting your tap water permanently is a lot of ongoing work. Choosing plants that suit it is free.
If you are seeing deficiency, work through the diagnosis properly before assuming pH is the cause. Iron symptoms on new growth with everything else healthy is the pattern to look for.
Common mistakes
Worth avoiding
- Dosing more iron into a high pH tank. The limit is availability, not quantity. All AquaLabs Fertilisers contain ample Iron.
- Using a pH down product without touching KH. The buffer pulls it straight back, and the swing is worse for livestock than the original pH ever was.
- Assuming all chelated iron is equivalent. EDTA in alkaline water is doing far less than the label suggests, and the strongest chelate is not automatically the best one.
- Chasing a pH number. pH matters because of what it does to availability, not as a target in itself.
- Reading pH once and assuming it is constant. In a CO2 tank it moves through the day by a full unit or more.
- Changing KH quickly. No more than 2 dKH in 24 hours, in either direction.
The short version
- Dosing a nutrient is not the same as making it available
- Macronutrients stay available across the whole normal pH range
- Iron, manganese, copper and zinc precipitate out as pH rises, and zinc also binds to surfaces
- Iron and phosphate precipitate each other regardless of pH, which is why unchelated fertilisers had to be split into separate bottles
- Chelates hold metals in solution and are what allows a complete all-in-one fertiliser to work
- Stability and uptake pull in opposite directions. EDTA releases most easily but gives up first, EDDHA holds hardest but is hardest to absorb
- EDTA works to about pH 6.5, DTPA to about 7.5
- AquaLabs fertilisers combine EDTA and DTPA iron to cover both ends, with EDTA copper, manganese and zinc
- We do not use EDDHA, because it is hardest for plants to absorb, does not blend stably, and stains the water
- pH is set by KH, so KH is the lever if you need to change it
- More iron does not fix a pH problem
Most fertiliser advice stops at what is in the bottle. What actually reaches the plant depends just as much on the water it lands in, and for the trace metals that comes down to pH and to which chelates were chosen to hold them there.