Liquid fertiliser for aquarium plants: choosing and dosing

1. What liquid fertiliser provides

Aquatic plants take nutrients up through both leaves and roots. In an aquarium with many fish, plants get some nutrients from fish waste, but some elements are often short, for example iron and other micronutrients. A liquid fertiliser supplements what the water and substrate do not provide.

Fertilisers fall broadly into two kinds. Macronutrients are nitrogen, phosphorus, potassium and magnesium: plants need them in larger amounts. Micronutrients, such as iron, manganese and others, are needed in smaller amounts, but a shortage shows quickly in the leaves. Some products combine them, others keep them apart.

For a beginner the easiest choice is usually one complete fertiliser dosed by the pack. Separate nitrogen and phosphorus products are meant for a more experienced aquarist who measures nitrate and phosphate and wants to adjust them precisely. Before buying your first fertiliser, check how many plants you actually have and how fast they grow.

Liquid fertilisers

2. Tropica Premium, Specialised and Carbon

Tropica Premium Nutrition is a liquid fertiliser without nitrogen and phosphorus. It is intended for aquariums with many fish and few or slow-growing plants. It contains iron, magnesium and micronutrients. The recommended dose is 6 ml (3 pushes of the pump) per 50 litres of water per week, added at the water change.

Tropica Specialised Nutrition contains nitrogen and phosphorus and is intended for aquariums with many fast-growing and demanding plants. Its dose is the same, 6 ml per 50 l per week. If algae appear, halve the dose and change water more often. Tropica Carbon Nutrition is liquid carbon: the recommended dose is 4 ml per 100 l of water per day, and it can be used alone or together with Tropica fertilisers.

3. Aquaforest and 2Hr options

Aquaforest Macro is a concentrated macronutrient fertiliser (nitrogen, phosphorus, potassium, magnesium): the dose is 5 ml per 100 l twice a week, recommended together with Aquaforest Micro and not to be combined with other macronutrient products. N Boost and PO4 Boost are in the range separately if you want to adjust a single element precisely. Aquaforest Red Boost supports the colour of red plants.

2Hr Aquarist APT Complete is an all-in-one liquid fertiliser: the dose is 5 ml per 100 l four times a week, or 3 ml daily. For new tanks with aquasoil and for aquariums with more fish the manufacturer recommends APT Zero, and APT EI is designed for the Estimative Index method, with a dose of 5 ml per 100 l three times a week. These products differ, so always follow the dose on your own pack.

4. Dosing and overdosing

The dose is always written on the pack and the product page. For a beginner it makes sense to start with the lower recommended dose on the pack and increase it only when the plants need it. Do not double the dose if plants look pale: the cause can also be light or CO2.

Too much fertiliser together with strong light gives algae an opening. Algae grow when there are more nutrients than plants can use. To avoid this, change part of the water weekly, do not add fertiliser between water changes without need, and do not add several products containing macronutrients at once.

Algae in the aquarium

5. Fertiliser, light and CO2

Fertiliser, light and CO2 work together. In a tank with low light and no CO2, plants grow slowly and need few nutrients, so a smaller dose is often enough. In a tank with high light and CO2, plants grow fast and consume more, so fertilising is more regular.

When you change one factor, watch the other two. If you raise the light, think about CO2 and fertiliser too. If you add CO2, make sure there are enough nutrients. To follow the changes, make them one at a time and note what changed and when.

A good habit is to keep a simple notebook: write down the fertiliser name, the dose and the water change date. After a couple of weeks you can see whether plant growth and leaf colour have improved, and you can adjust the dose calmly.

Liquid fertiliser comparison (from manufacturer descriptions)

Product	Contains	Suits	Dose on pack
Tropica Premium Nutrition	Iron, magnesium, micronutrients; no N or P	Many fish, few plants	6 ml / 50 l weekly
Tropica Specialised Nutrition	N and P, iron, magnesium, micronutrients	Many fast plants	6 ml / 50 l weekly
Tropica Carbon Nutrition	Liquid carbon	Plants, helps prevent algae	4 ml / 100 l daily
Aquaforest Macro	N, P, K, Mg	With Aquaforest Micro	5 ml / 100 l twice weekly
2Hr APT Complete	All-in-one fertiliser	Planted aquariums	5 ml / 100 l 4× weekly
2Hr APT Zero	All-in-one fertiliser	New aquasoil, more fish	5 ml / 100 l 4× weekly