DIY aquarium fertiliser: the honest, no-fear guide to mixing your own

“I’m not a chemist” — and you don’t need to be

Almost everyone who mixes their own aquarium fertiliser felt nervous the first time. The salt names look intimidating. There is talk of solubility, chelates and cation antagonism. It feels like the sort of thing that goes wrong and wipes out your fish.

Here is the honest truth, and the whole point of this guide: DIY dosing is one of the safest, simplest and most-established things you can do in this hobby. It has a fifty-year track record. The maths behind it is arithmetic, not chemistry. And the very worst realistic outcome of a beginner’s mistake is usually a little algae — fixed by a water change — not a dead tank.

Let’s take the five common fears and deal with them one at a time, backed by the actual science, and end with a recipe you can copy tonight.

What aquarium fertiliser actually is

Strip away the branding and every planted-tank fertiliser — the £22 premium bottle and the home-made jar alike — is doing exactly one thing: putting a handful of dissolved ions into your water for the plants to absorb.

Plants need the same short list of nutrients whether they grow in a field or a fish tank. The big three (“macros”) are nitrogen, phosphorus and potassium — the N, P and K you see on any plant food. Alongside them come the “micros” or trace elements: iron, manganese, boron, zinc, copper and molybdenum, needed in tiny amounts (plus a couple more, such as chlorine and nickel, that your water and fish food supply without you ever thinking about them). Which nutrients plants require is textbook plant physiology, not opinion [1].

The crucial insight — the one that dissolves most of the mystique — is this: a nitrate ion is a nitrate ion. The nitrogen your plant takes up is identical whether it came from a £22 bottle of a boutique fertiliser or from a few pence of potassium nitrate you dissolved yourself. Plants absorb ions; they have no way of knowing, and no reason to care, which brand delivered them.

So “making your own fertiliser” is not a chemistry experiment. It is buying the same ingredients the bottle already contains, and dissolving them in water. That is the entire trick.

The five salts that do almost everything

You do not need a laboratory’s worth of chemicals. A complete, flexible dosing kit is just a handful of powders, most sold as garden or hydroponics supplies:

Salt	What it delivers	Notes
Potassium nitrate (KNO3)	Nitrogen (as nitrate) + a lot of potassium	Your main N source. Two-for-one: it supplies a big chunk of K for free.
Monopotassium phosphate (KH2PO4)	Phosphorus (phosphate) + a little potassium	You use tiny amounts — a small bag lasts years.
Potassium sulphate (K2SO4)	Potassium	Tops up K beyond what the nitrate and phosphate already give.
CSM+B trace mix	Iron + manganese, boron, zinc, copper, molybdenum	An all-in-one micronutrient blend. This is your “trace bottle”.
Epsom salt (MgSO4·7H2O)	Magnesium	Only needed for soft or RO water; hard water usually has plenty.

That’s it. Three macro salts, one trace blend, and an optional magnesium salt for soft-water keepers. From those you can hit any dosing level you like, from a gentle low-tech feed to a full high-energy regime. If you want the deeper detail on what each salt does and how the trace blend is chelated, our Make Your Own Fertiliser tool includes a plain-English salt-and-chelate reference alongside the recipe.

The science, in plain English

You do not need any of this to dose successfully — the calculator handles it — but understanding the three ideas below is what turns nervousness into confidence.

1. Dissolving: only one salt is fussy

Each salt has a solubility limit — the maximum that will dissolve in a given volume of water. Most of the salts above are extremely soluble and dissolve without a thought. Potassium nitrate dissolves to around 310 g per litre at room temperature; monopotassium phosphate to around 220 g/L [2].

The one to respect is potassium sulphate, which only dissolves to about 110 g/L at 20 °C — and less in a cold room. In practice this simply means: don’t try to cram too much into too small a bottle, use lukewarm rather than cold water, and stir until it’s clear. Salts that share an ion crowd each other a little — your three macro salts all carry potassium, so each dissolves slightly less than it would on its own (the “common-ion effect”) — which is another reason to keep your solution comfortably below the limit rather than saturated. A well-designed recipe leaves a wide margin so this is never a problem — you should never see undissolved powder sitting in the bottom of the bottle.

2. Chelation: why the trace bottle is “caged”

Iron and the other micronutrients have a habit of reacting with oxygen and phosphate and dropping out of solution as particles the plant can’t use. To stop that, trace mixes come chelated — each metal ion is wrapped in a molecular “cage” that keeps it dissolved and available.

The type of cage sets how high a pH it survives. EDTA (what most CSM+B uses) holds iron up to about pH 6.5; DTPA to around 7.5; EDDHA to around 9 [3]. For the great majority of soft-to-neutral planted tanks, and especially CO₂-injected tanks running below pH 7, standard EDTA-chelated CSM+B is perfectly adequate. Only in genuinely hard, alkaline water does it pay to add a little DTPA or EDDHA iron to the trace bottle. If new leaves ever come in pale or yellow despite dosing, our guide to diagnosing nutrient deficiencies covers what to look for.

3. Nitrogen: we keep it simple and safe

Plants can take up nitrogen as nitrate (NO₃⁻) or as ammonium/urea. Our approach — and the classic DIY approach — uses potassium nitrate, so all the nitrogen is nitrate. Nitrate is the safe, forgiving form: it does not become toxic ammonia, it is hard to overdose into trouble, and it is what the entire Estimative Index method was built around.

“Will I poison my fish or shrimp?”

This is the fear that stops most people, so let’s meet it head-on.

The dominant DIY method, the Estimative Index (EI), was designed specifically so that you cannot easily get it wrong. Its logic, popularised by Tom Barr, is deliberately un-fussy: dose a small, deliberate surplus of every nutrient so plants are never short, and then reset any build-up with a large weekly water change [4]. Because the whole method is built on generous margins, being 10% or even 30% out on your macro figures changes nothing you could see. You are aiming at a wide target, not threading a needle.

The nutrients themselves are, at planted-tank doses, very benign. Nitrate only becomes a concern for hardy community fish at concentrations far above anything a sane dosing routine produces, and the weekly water change keeps it in check. (Sensitive species, fry and shrimp tolerate less, so lean the dose and keep the water changes up in those tanks.) Phosphate and potassium are tolerated across a very wide range — potassium in particular is remarkably forgiving, as our guide to potassium build-up explains. Iron and traces are dosed in fractions of a part per million.

There is one honest caveat, and it is about shrimp, not fish. Trace mixes contain a little copper, which invertebrates are sensitive to. At normal dosing levels this is well within the safe range and countless shrimp tanks are dosed with CSM+B without issue — but it is the one ingredient to respect. Don’t massively overdose traces in a shrimp-focused tank, and if in doubt, dose traces a little leaner. That is the extent of the danger.

Making your first bottle, step by step

The mechanics are genuinely trivial. You will split your dosing into two bottles — a macro bottle and a trace bottle — because iron and phosphate slowly react if stored together in one concentrated mix. Then:

1. Get the salts. In the UK they’re sold by aquarium fert suppliers and hydroponics/garden shops. A full starter kit — potassium nitrate, monopotassium phosphate, potassium sulphate and a CSM+B trace mix — costs around £30–35, and because you use tiny amounts, most of those bags last one to several years.
2. Work out the grams. This is the only “maths”, and you don’t have to do it — a calculator turns your tank size and chosen level into exact weights (more below). Or copy the worked example in the next section.
3. Mix the macro bottle. Weigh the macro salts on a cheap 0.01 g jeweller’s scale, tip them into a bottle of RO or distilled water, and stir with lukewarm water until fully clear.
4. Mix the trace bottle the same way, in its own smaller bottle. Keep it in a cool, dark place — micros degrade — and make a fresh batch every couple of weeks.
5. Dose. Add the recommended millilitres to your tank on your chosen days. Do your weekly water change. That’s the whole routine.

A set of cheap 0.01 g scales and two clean bottles are the only equipment you need to buy beyond the salts themselves.

A real recipe for a 200 litre tank

To make this concrete, here is a complete “Standard” recipe for a 200 litre planted tank, dosed daily — the kind of balanced feed that suits most tanks with moderate light. These are real figures you could mix tonight.

Bottle	Mix into	Salts	Dose
Macro bottle	500 ml RO/distilled water	KNO3 36 g · KH2PO4 3 g · K2SO4 41 g	~6.5 ml daily
Trace bottle	~90 ml RO/distilled water	CSM+B 0.9 g	~6.5 ml daily, remade every 2 weeks

That macro bottle lasts roughly two and a half months; the trace bottle is a small, fresh batch. Between them they deliver, each week, about 10 ppm nitrate, 1 ppm phosphate, 15 ppm potassium and 0.15 ppm iron — comfortably inside the healthy range for a standard planted tank. Change your tank size, water or dosing level and the numbers change, which is exactly what the calculator is for.

What it actually costs

This is the part that makes people wish they’d switched sooner. Here is the cost of feeding that same typical 200 litre tank for a year, comparing DIY against a budget all-in-one and a premium one, at each product’s recommended dose (UK prices, mid-2026):

Method	Roughly per year	What you’re paying for
DIY raw salts	~£16	The salts themselves, nothing else.
TNC Complete (budget all-in-one)	~£26	Convenience of one pre-mixed bottle.
APT Complete / APT3 (premium)	~£155	A premium chelate blend and brand.

So DIY comes in at roughly a tenth of the premium bottle and comfortably under even the budget all-in-one — and that’s before accounting for the fact that your ~£30–35 starter kit stocks you for one to several years. A 90 g bag of monopotassium phosphate, for a tank this size, lasts the better part of a decade. Put another way: a year of DIY ferts can cost less than a single bottle of the premium stuff.

A fair note in the other direction: a premium all-in-one does buy you real things — a superior iron chelate that holds up in hard water, added co-factors, and the genuine convenience of a single bottle you never have to weigh out. Whether that’s worth a 10× premium is your call. But it is a convenience-and-brand premium, not a nutrition one — your plants get the same ions either way.

When DIY isn’t worth it

To keep this honest — because a guide that only ever says “do it” isn’t trustworthy — here’s when a bottle genuinely makes more sense:

• Very small tanks. On a nano tank the weekly weights become tiny fractions of a gram that are fiddly to measure accurately. A ready-made liquid is easier at that scale.
• You value convenience over cost. If the ten minutes of mixing every couple of months genuinely isn’t worth £140 a year to you, that’s a perfectly rational choice. No judgement.
• A quiet low-tech tank. A gently-planted, low-light tank with fish may need so little added fertiliser that a cheap liquid, dosed occasionally, is all the hassle it warrants.

For everything in between — the average planted community tank, and every high-energy setup — DIY is cheaper, more flexible, and no harder once it’s set up.

Let the calculator do the maths

The single thing that puts people off is the weighing-and-ratios step — and that’s the one thing you can hand off entirely. AquaCalc’s Make Your Own Fertiliser calculator asks for your tank size, how often you want to dose, and which level you’re aiming at (lean, standard or high-tech), and hands you a complete recipe: exactly how many grams of each salt to weigh, how much to dose, how long each bottle lasts, and a check that every nutrient lands in the healthy range. It will even subtract the nutrients your tap water already provides.

Mixing your own fertiliser is one of those hobby steps that feels like a leap and turns out to be a small hop. The chemistry is friendlier than it looks, the method was built to forgive mistakes, and the savings are genuinely large. Don’t let the salt names scare you off.

1. Marschner, P. (ed.) (2012). Marschner’s Mineral Nutrition of Higher Plants, 3rd ed. Academic Press. (Essential plant macro- and micronutrients; nitrogen uptake as nitrate and ammonium.) ISBN 978-0-12-384905-2.
2. Haynes, W.M. (ed.) (2016). CRC Handbook of Chemistry and Physics, 97th ed. CRC Press. (Aqueous solubility of potassium nitrate, monopotassium phosphate and potassium sulphate.) ISBN 978-1-4987-5428-6.
3. Norvell, W.A. (1972). Equilibria of metal chelates in soil solution. In Micronutrients in Agriculture, Soil Science Society of America, pp. 115–138. (pH stability of EDTA, DTPA and EDDHA metal chelates.)
4. Barr, T. (2005 onward). The Estimative Index of dosing, or No Need for Test Kits. Barr Report. (Origin and rationale of surplus dosing with weekly water changes — hobby method, not peer-reviewed literature.)