Aquarium Plant Fertiliser Dosing: A Beginner's Guide

How aquarium plant fertilisers work: macronutrients vs micronutrients, water-column liquids vs root capsules, when to start dosing, and how to avoid the overdosing that feeds algae.

What plant fertiliser actually does

Light and CO2 are only part of the picture. Aquatic plants also need mineral nutrients, and most of these must come from fertiliser. Tropica lists nitrogen, phosphorus, iron and manganese among the nutrients plants take up in larger amounts through both roots and leaves, plus micronutrients such as copper, molybdenum, zinc and boron that act as enzyme components. A shortage of any of them slows growth and shows up as visible deficiency symptoms.

Macronutrients vs micronutrients

Macronutrients (nitrogen N, phosphorus P, potassium K, plus magnesium and calcium) are used in comparatively large quantities. Micronutrients or trace elements (iron, manganese, zinc, copper, molybdenum, boron) are needed only in tiny amounts but are still essential. Many liquid fertilisers split these into a 'macro' bottle and a 'micro/trace' bottle so you can adjust them separately. As a reference point, the 2Hr Aquarist describes a macro mix delivering roughly 3 ppm nitrate, 1 ppm phosphate, 4 ppm potassium and 1 ppm magnesium per dose, and a trace mix delivering about 0.1 ppm iron plus proportionate traces.

Water-column liquids vs root capsules

Liquid fertiliser dissolves into the water column and is absorbed through the leaves. Tropica notes it needs frequent dosing (daily or weekly) and works for every plant type, including mosses and floating plants. Root capsules are pushed into the substrate and release their nutrients over roughly 6-9 months; Tropica recommends them for large, sturdy rooted plants and established carpets, not for plants without substrate roots.

When to start and how much

1. Start dosing from day one of planting — the 2Hr Aquarist advises fertilising immediately, since plants absorb nutrients through their leaves straight away.
2. Dose small and often rather than one big weekly slug, and follow the bottle's rate for your tank volume.
3. Scale nutrients to demand: more light, CO2 and plant mass means the plants can use more fertiliser.
4. Do a weekly water change to reset nutrient levels and remove excess.

Sources: Tropica Plant Guide, Fertiliser and CO2 (tropica.com); Tropica Nutrition Capsules (tropica.com); The 2Hr Aquarist, Planted-tank fertilizing 101 and 'how much to dose' (www.2hraquarist.com www.2hraquarist.com).