Aquarium Fertiliser Calculator

The Aquarium Fertiliser Calculator computes weekly macro and micro doses for planted freshwater aquariums using three established methodologies — Estimative Index, PPS-Pro, and lean dosing — plus an estimated weekly cost in dry-salt form.

The Counterintuitive Truth About Algae

The most persistent myth in planted aquariums is that algae is caused by excess fertiliser. Beginners develop algae, read that fertilisers "feed the algae", and reduce dosing — which makes the problem worse. The underlying reality is almost the opposite: algae appears when any single nutrient (nitrogen, phosphate, potassium, iron, or CO₂) becomes limiting, because algae adapts to low-nutrient conditions faster than plants. Tanks running full EI doses with adequate CO₂ and a healthy plant mass routinely show nitrate at 20-30 ppm and phosphate at 2-3 ppm with zero visible algae.

Underfertilising causes algae because plants slow down, fail to outcompete algae for whatever is abundant, and surrender territory. The corrective action in a weakly-planted algae-prone tank is usually more fast-growing plants plus continued full dosing, not dose reduction. This is the key insight that separates experienced planted-tank keepers from beginners who interpret every algae outbreak as a signal to dose less.

Three Methods, Different Philosophies

EI, PPS-Pro, and lean dosing all work, but they rest on different assumptions about how much fertiliser a tank needs and how water changes interact with dosing.

The Estimative Index, developed by Tom Barr in the early 2000s, doses nutrients to consistent weekly excess and relies on a 50% weekly water change to reset levels before the next dosing cycle. Weekly targets are NO₃ 20 ppm, PO₄ 3 ppm, K 30 ppm, and Fe 0.5 ppm. EI is forgiving — you cannot easily underdose — and is the standard for aggressive planted tanks with dense stems and carpet growth. The trade-off is higher weekly cost (still under £1 in dry salts for most tanks) and a requirement for disciplined weekly changes. Skip a change and nutrient accumulation can trigger algae within 2-3 weeks.

PPS-Pro, developed by Edward Venter, doses daily to match plant consumption rate. Targets are lower — NO₃ 7 ppm, PO₄ 1 ppm, K 10 ppm, Fe 0.2 ppm — because the intent is to replace what plants take up rather than maintain excess. PPS-Pro allows smaller or less-frequent water changes because there is no excess to reset, but demands more discipline on a daily basis. It suits precision-minded keepers who enjoy the dosing ritual and are comfortable with smaller per-dose amounts.

Lean dosing targets the minimum viable nutrient level — around NO₃ 5 ppm, PO₄ 0.5 ppm — and is the approach for shrimp-heavy tanks where excess phosphate and trace mixes can harm Neocaridina or Caridina colonies. Low-tech planted tanks (low light, low plant density, no CO₂) also benefit from lean dosing; full EI doses in a low-demand tank drive excess that plants cannot consume, creating the classic underlying-imbalance algae pattern described above.

Dry Salts vs Liquid Fertilisers

Every branded aquarium fertiliser bottle contains the same compounds the calculator recommends — potassium nitrate (KNO₃), mono-potassium phosphate (KH₂PO₄), potassium sulphate (K₂SO₄), and chelated iron/trace mix. The difference is concentration and markup. A £12 bottle of Seachem Nitrogen contains maybe 3 g of active KNO₃ per 500 mL; a £12 bag of pure KNO₃ holds 1 kg. The cost-per-gram difference is roughly 300×.

Dry salts require three things: a 0.01 g jewellery scale (£15), clean storage jars, and a willingness to dissolve salts in distilled water if doses are too small to weigh directly. For a 60 L tank on EI, the weekly KNO₃ dose is around 2 g — weighable directly but also cleanly dosable as 200 mL of a 10 g/L solution. Under 30 L, dosing by solution volume is essentially required because dry weights drop below the precision of household scales.

The calculator's cost-per-week output reflects dry-salt economics — typically £0.20-0.80 per week for tanks up to 200 L. At annual scale, a £20 investment in salts covers most planted tanks for 12+ months. The liquid-bottle equivalent would cost £100-200 for the same coverage. This is the strongest single argument for moving from branded liquids to dry dosing once a tank passes 60 L.

Plant Mass Drives the Multiplier

The plant mass multiplier is the calculator's most important variable for matching dose to actual consumption. Low plant mass (sparse planting, slow growers) runs at 0.5× the base target — a fully-dosed EI regime in a low-plant tank produces excess that plants cannot use, which encourages algae. Medium plant mass (standard planted community) runs at 1.0× base. High plant mass (dense stems, carpet coverage, aggressive growth) runs at 1.5× base because the larger leaf surface and higher photosynthesis rate drive higher nutrient uptake.

The multiplier also interacts with lighting and CO₂. High plant mass without adequate light or CO₂ target does not consume nutrients faster — plants cannot photosynthesise without all three inputs. If you have dense planting but low light or no CO₂, run medium-mass dosing rather than high until you add lighting or injection. The calculator does not account for this interaction directly; it assumes the keeper has matched lighting and CO₂ to the stated plant density.

The Weekly Water Change Reset

EI dosing assumes a 50% weekly water change as the nutrient reset. The calculator's waterChangeFrequency input adjusts doses downward when keepers run smaller or less-frequent changes — -25% for bi-weekly 50% changes, -15% for weekly 30% changes. The reasoning is that less water replacement means less nutrient removal, so accumulating doses would drive NO₃ and PO₄ into algae-feeding territory over 4-6 weeks.

For keepers not running the standard EI water change schedule, PPS-Pro is the better fit — its daily dosing pattern is designed around smaller changes. Lean dosing works with any change schedule because the target nutrient levels stay below the threshold where water changes become the primary removal mechanism. Align your dosing choice with your maintenance discipline, not the other way around. Pair this with a consistent water change percentage that matches your tank's stocking and plant load.

Watching for Deficiencies (Carefully)

Plants signal nutrient deficiencies through specific leaf symptoms. Nitrogen deficiency appears as yellowing on older leaves — the plant mobilises nitrogen from mature leaves to support new growth. Iron deficiency appears as yellowing on new leaves — iron is not mobile, so the plant cannot redirect existing stores. Potassium deficiency produces pinholes in leaves, often ragged and progressing from leaf edges inward. Phosphate deficiency shows as dark necrotic spots and slow new growth.

The caution is that deficiency symptoms rarely appear alone — most planted tanks have a CO₂ limitation or flow problem that masquerades as nutrient deficiency. Adding more fertiliser to a CO₂-limited tank does not help; it often produces algae because the unused nutrients stay in the water column. Check CO₂ drop-checker colour, flow distribution, and lighting duration before concluding the tank is nutrient-deficient. Experienced keepers develop an instinct for this sequence — the beginner reflex is to add fertiliser at the first yellow leaf, which is the wrong move 70% of the time.

Accuracy and Scale

For tanks below 30 L, absolute dose precision matters because the margin between useful and excessive is narrow. A 0.1 g error on KH₂PO₄ is a meaningful percentage of the weekly target. For tanks above 100 L, the margins widen and precise ±0.01 g weighing is less critical. Either way, the best workflow is dissolving dry salts in distilled water (500 mL per week's worth) and dosing by millilitre — both more accurate and less fiddly than dry dosing.

Precision matters here the way it matters in canine feeding precision — small consistent errors compound into nutritional problems over weeks, while occasional ±10% variation is essentially invisible. Adjacent domains like raw feeding dosing parallels rely on the same "measure, then dose" discipline. A well-maintained tank aligned with its stocking balance responds predictably to fertiliser changes; an understocked or overstocked tank makes every dosing adjustment harder to interpret.