Can Plant Fertilizer Kill Indoor or Outdoor Fish?

There are various organic and chemical plant fertilizers available. Both types of fertilizers contain nutrients that are crucial to plant growth. However, excessive nutrients can be harmful to aquatic life.

Plant fertilizers can kill indoor or outdoor fish. But those that are specialized for aquarium and pond plants are harmless to indoor and outdoor fish when applied in the recommended dose. So fish owners should ensure only to use fertilizers designed for aquarium and pond plants.

This article will further discuss how generic plant fertilizers endanger indoor and outdoor fish. It’ll also provide recommendations for aquarium and pond plant fertilizers you can use. So keep reading!

Can I Use Regular Plant Fertilizer in My Fish Tank or Pond?

The best plant fertilizers on the market perform exceedingly well with terrestrial plants. However, when used in plants in an aquatic environment, the nutrients they release can affect the water quality.

You should not use regular plant fertilizer in your fish tank or pond because it harms aquatic ecosystems by causing chemical imbalances. For instance, most plant fertilizers contain ammonia, which in excess could kill both your aquarium plants and fish.

The water quality in your aquarium is essential for plant growth and fish health. Therefore, you should monitor your fish tank’s pH, ammonia, nitrate, and nitrite levels and change the water regularly.

Let’s get more into why you should not use regular plant fertilizers in your aquarium or fish pond.

  • The water quality may decrease. Aquarium life, including water plants and fish, thrives in a regulated environment. High pH and an increase in chemicals can impact the water quality, making it toxic to fish. Additionally, not changing the water often enough can cause the nitrate level to rise and pH to drop significantly.
  • Algal blooms. A spike in nutrients in the water can also increase algal growth. When algae grow out of control, it causes a decrease in the amounts of dissolved oxygen in water, suffocating your fish and plants. Moreover, some types of algal blooms release toxins when they decay or degrade, harming the fish.
  • Fish and plants are very sensitive to changes in their water. Inappropriate chemical composition of water caused by plant fertilizer can stress your fish, causing them to become sick or die.

Importance of Fertilizer for Your Aquarium Plants

Like terrestrial plants, aquatic plants need essential nutrients to grow healthy. These nutrients fall into two major groups: 

  • Macronutrients: plants require these nutrients in large amounts. Macronutrients are further defined as primary or secondary. The primary nutrients constitute nitrogen, phosphorus, and potassium, while the secondary nutrients are calcium, magnesium, and sulfur.
  • Micronutrients: plants use these nutrients in small quantities. They include boron, copper, chlorine, iron, manganese, molybdenum, and zinc.

Aquatic plants absorb some of these nutrients through the water column or substrate.

Common Aquatic Plant Nutrient Deficiencies

It’s crucial to supplement macronutrients and micronutrients by adding plant fertilizer to prevent nutrient deficiencies.

Here are common aquarium plant nutrient deficiencies, how to identify them, and how to fix them:

  • Calcium deficiency: new leaves grow out in a twisted manner, and the tips of the leaves may wither. A calcium deficiency often coincides with water with a lower general hardness (GH), such as reverse osmosis water. To increase calcium levels, gradually increase your water’s hardness by adding crushed coral in the filter or atop the substrate.
  • Iron deficiency: older leaves appear normal, but newer leaves display yellowing or paleness with veins that remain dark. Treat iron deficiency in aquatic plants using fertilizers rich in iron.
  • Magnesium deficiency: older leaves become lighter in color, yet their veins remain dark. There may be drooping in the edges as well. Supplement the nutrient using an all-in-one aquatic plant fertilizer or a magnesium-based fertilizer.
  • Nitrogen deficiency: old leaves turn yellow and translucent, starting at the tips. Leaves may also start to fall off, beginning with the oldest leaves. A nitrogen deficiency may be due to not adjusting the dose as your plants grow. Feed your plants more as they become taller and lush.
  • Phosphate deficiency: older leaves start turning yellow with soggy brown-like patches. Algae may also form on the surface of the dying leaves, new leaves will usually grow shorter, and the stems will appear much thinner. Phosphate-absorbing pads in the fish tank may combat algae but will also starve your plants of phosphate.
  • Potassium deficiency: brown or yellow-rimmed pinholes on the leaves and edges of leaves may curl inward and appear pale. You can correct potassium deficiency by simply upping the dose of your all-in-one fertilizer.

Fertilizing Aquarium Plants

Fertilizers can either be complete or incomplete based on the nutrients they contain. I recommend getting a complete fertilizer as it provides the three most essential plant nutrients, namely nitrogen, phosphorus, and potassium.

However, regular plant fertilizers may contain high percentages of nutrients like nitrogen, which can be deadly to indoor fish if in high concentrations. So play it safe and only use plant fertilizers specially made for use in a fish tank.

Additionally, the right fertilizer can prove lethal to plants and fish alike if applied in excess. So only add doses according to the fertilizer label to prevent increasing the nutrients in your aquarium or fish pond to lethal levels.

Fertilizing Fish Pond Plants

In addition to making your aquatic plants grow strong and healthy, pond plant fertilizers also increase fish yields. The harvest of a fertilized pond can be three to four times that of an unfertilized pond.

Fertilizing fish ponds increases the availability of nitrogen, phosphorus, and potassium. Nutrient enrichment, in turn, stimulates the growth of planktonic algae (microscopic plants), which are the base of the food chain in a pond. Zooplankton (microscopic animals) feed on the planktonic algae and, in turn, are fed on by small fish.

However, excess nitrogen and phosphorus in the pond can lead to algal blooms. Consequently, dissolved oxygen content reduces, causing your outdoor fish to die of suffocation. So you should only use fertilizers specially formulated for aquatic pond plants.

Since phosphorus is usually the limiting nutrient in fish ponds, the formulation of pond fertilizers typically contains little nitrogen and no potassium. A limiting nutrient is a nutrient that is present in the least quantity. Therefore, phosphate fertilizers are generally the go-to inorganic fertilizers for fish ponds.

You can also use organic fertilizers, such as animal manure, but it’s best not to. The amount of organic fertilizer needed to achieve the same nutrient input as a small amount of chemical fertilizer is significantly large. High application rates of organic fertilizers increase the likelihood of dissolved-oxygen depletion, which means stress and possibly death for your pond fish.

Chemical fertilizers are available in various forms: liquid, tablets, spikes, and granules. Whatever variety you choose, follow the instructions on the label to the letter since the proper balance of nutrients in an aquatic ecosystem is vital to the growth and survival of aquatic life.

Final Thoughts 

Plant fertilizer provides your aquatic plants with the essential nutrients needed for healthy plant growth. But be careful of the fertilizer you use and the dosage you give, as excessive nutrients can kill fish and other aquatic life.

For instance, using regular plant fertilizers in your fish tank or pond can cause algal blooms and, in turn, a deficiency of dissolved oxygen. Instead, use specialized fertilizers for aquarium plants or pond plants, of which there are various forms.

Aquatic plant fertilizers are safe for indoor and outdoor fish when applied in the recommended doses. Otherwise, your plants and fish may both die.

Alexander Picot

Alexander Picot is the principal creator of, a website dedicated to gardening tips. Inspired by his mother’s love of gardening, Alex has a passion for taking care of plants and turning backyards into feel-good places and loves to share his experience with the rest of the world.

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