Do Aquarium Plants Need Fertiliser? A Simple Answer

Table of Contents

• The Short Answer
• What Aquarium Plants Actually Need
• What Nutrients Come From Fish
• What You Need to Supplement
• Low-Tech Tanks With Fish
• High-Tech Tanks With CO2
• Inert vs Active Substrate
• Types of Aquarium Plant Fertiliser
• Frequently Asked Questions

“Do I need to add fertiliser, or will fish waste take care of it?” It is one of the most common questions we hear at Gensou. The short answer: most planted tanks benefit from some form of fertilisation, and many require it. This guide breaks down which nutrients fish provide and which you need to supplement.

The Short Answer

Here is a quick reference based on your tank type:

What Aquarium Plants Actually Need

Aquatic plants require three categories of nutrients to grow:

Macronutrients (Needed in Large Quantities)

• Nitrogen (N) — usually available as nitrate (NO3-) from fish waste
• Phosphorus (P) — available as phosphate (PO4) from fish waste and food
• Potassium (K) — rarely supplied naturally; almost always needs supplementing

Micronutrients / Trace Elements (Needed in Small Quantities)

• Iron (Fe) — critical for chlorophyll production; deficiency causes yellowing
• Manganese (Mn) — essential for photosynthesis
• Boron (B), Zinc (Zn), Copper (Cu), Molybdenum (Mo) — needed in tiny amounts

Non-Mineral Requirements

• Carbon (C) — the most important nutrient; supplied by CO2 (either injected or from fish respiration/atmosphere)
• Light — the energy source driving photosynthesis

Growth is limited by whichever nutrient is in shortest supply (Liebig’s Law of the Minimum). A deficiency in just one nutrient limits growth and often triggers algae.

What Nutrients Come From Fish

Fish waste and uneaten food provide certain nutrients through biological breakdown:

The key takeaway: fish provide nitrogen and phosphorus reasonably well, but potassium and iron are almost always deficient. These two nutrients are the most common limiting factors in planted tanks that rely solely on fish waste.

What You Need to Supplement

Potassium: The Most Commonly Deficient Nutrient

Potassium is barely present in fish waste and not reliably supplied by Singapore’s PUB tap water. It is almost always the first nutrient to become deficient. Symptoms include pinholes in older leaves, yellowing margins and stunted growth.

Iron: Essential for Green, Vibrant Growth

Iron is critical for chlorophyll production. Without it, new leaves emerge pale or yellow (chlorosis). Red plants are especially heavy iron consumers. Iron readily oxidises in water, so chelated formulations that remain plant-available longer are preferred.

Low-Tech Tanks With Fish

In a low-tech planted tank (no CO2 injection, low to moderate lighting) with a reasonable fish population, the fertilisation approach can be relatively simple:

• Fish waste handles nitrogen and phosphorus. In a moderately stocked tank, these two macronutrients are typically supplied in adequate quantities by the fish themselves.
• Add a liquid micro-nutrient fertiliser containing iron and trace elements 2-3 times per week. This fills the gap that fish waste cannot cover.
• Supplement potassium either through a comprehensive liquid fertiliser that includes it or a dedicated potassium supplement.
• Root tabs for heavy root feeders. Swords (Echinodorus), Cryptocoryne and Vallisneria feed primarily through their roots. Pushing nutrient-rich root tabs into the substrate near their roots every 2-3 months ensures they receive adequate nutrition at the root zone.

For recommended products and dosing guidance, see our liquid fertiliser aquarium guide.

High-Tech Tanks With CO2

With CO2 injection and high lighting, nutrient demand increases dramatically. Comprehensive fertilisation is essential.

Estimative Index (EI) Dosing

The most popular method involves adding excess nutrients throughout the week and performing a 50% water change to reset levels. A typical EI schedule:

• Day 1, 3, 5: Macronutrients — potassium nitrate (KNO3), potassium phosphate (KH2PO4), potassium sulphate (K2SO4)
• Day 2, 4, 6: Micronutrients — chelated iron and trace element mix
• Day 7: 50% water change to prevent nutrient accumulation

Lean Dosing

An alternative approach where nutrients are added in more controlled, lower amounts. This requires more careful monitoring (ideally with test kits for nitrate, phosphate and iron) but avoids the large weekly water changes required by EI. Lean dosing is popular with experienced hobbyists who have dialled in their specific tank’s consumption rates.

Inert vs Active Substrate: A Critical Distinction

Inert Substrates (Gravel, Sand, Lava Rock)

Inert substrates contain no nutrients of their own. They are purely structural — providing an anchoring medium for plant roots but nothing more. Tanks with inert substrates are entirely dependent on:

• Liquid fertilisers in the water column
• Root tabs pushed into the substrate near plant roots
• Fish waste that settles into the substrate over time

Without supplementation, plants in inert substrates will eventually show deficiency symptoms, particularly root-feeding species.

Active Substrates (ADA Amazonia, Tropica Soil, UNS Controsoil)

Active substrates are nutrient-rich soils designed specifically for planted aquariums. They come pre-loaded with nitrogen, phosphorus and various trace elements, and they buffer water to slightly acidic pH levels that most aquatic plants prefer.

Key points about active substrates:

• Nutrients are finite. Most active substrates supply nutrients for approximately 12-24 months before becoming depleted. After this period, supplementation with root tabs and liquid fertiliser becomes necessary.
• Initial ammonia release. New active substrates leach ammonia during the first few weeks, which is why fishless cycling is recommended before adding livestock.
• They still benefit from liquid micro-nutrients even when fresh, as the substrate primarily supplies nitrogen and some trace elements but may lack sufficient iron and potassium in the water column.

Types of Aquarium Plant Fertiliser

Frequently Asked Questions

Will adding fertiliser cause algae?

This is the most persistent myth in planted tank keeping. Fertilisers do not cause algae — imbalances cause algae. If you add iron but your plants lack CO2 or light to utilise it, the excess iron may contribute to algae. The solution is not to withhold fertiliser but to ensure all growth factors (light, CO2, nutrients) are balanced. In fact, nutrient-deficient plants are more susceptible to algae growth on their weakened leaves.

Can I use garden fertiliser for my aquarium?

No. Garden fertilisers are formulated for terrestrial plants and often contain compounds that are toxic to fish and invertebrates, including urea, ammonium salts and heavy metals. Some also contain herbicides or pesticides. Always use fertilisers specifically formulated for aquarium use.

How do I know if my plants need more fertiliser?

Common deficiency signs include yellowing leaves (iron or nitrogen), pinholes in leaves (potassium), stunted growth, pale new growth (iron) and melting or dropping older leaves (mobile nutrient deficiency — nitrogen, phosphorus, potassium or magnesium). If your lighting and CO2 are adequate and plants are still underperforming, insufficient fertilisation is the most likely culprit. Our liquid fertiliser guide covers specific deficiency symptoms in detail.

I only have Java fern and Anubias — do these need fertiliser?

Java fern and Anubias are among the least demanding aquarium plants. In a tank with fish, they often grow adequately with no supplementation at all. However, even these hardy species benefit from occasional iron and potassium dosing — you will notice darker, healthier green coloration and more robust growth. Since they are epiphytes (attached to hardscape rather than planted in substrate), they feed exclusively from the water column, making liquid fertiliser the appropriate delivery method.

Get Your Plants Thriving

Whether you are running a simple low-tech tank with a few easy plants or a full high-tech aquascape with demanding species, the right fertilisation approach makes all the difference between surviving plants and thriving ones. Visit us at Gensou, 5 Everton Park to browse our range of fertilisers, substrates and supplements. With over 20 years of planted tank experience in Singapore, we can recommend the perfect fertilisation strategy for your specific setup.

Related Reading

• Boron Deficiency in Aquarium Plants: Twisted Tips and Stunted Growth
• Calcium Deficiency in Aquarium Plants: Twisted New Growth
• How to Mix Dry Fertiliser for Your Planted Aquarium
• Magnesium Deficiency in Aquarium Plants: Symptoms and Solutions
• Manganese Deficiency in Aquarium Plants: Pale Patches Between Veins