Is Lawn Fertilizer Poisonous to Pond Fish?

Ponds are often in the line of fire when it comes to lawn fertilizer washing in after a rain. Owners must be cautious in using high-nutrient fertilizers as they can be indirectly harmful to your fish.

Phosphorus

Most fertilizers contain very high levels of phosphorus which is an essential nutrient to growing green, hearty grass and plants. Fertilizer that is not washed into the ground by rain eventually drains into the low point of your property--your pond.

Algae Blooms

Algae loves phosphorus from fertilizers

Excessive levels of phosphorus can induce algae blooms which can shade out beneficial vegetation and decrease the aesthetic value of your pond. Large algae blooms consume high levels of oxygen at night and spike pH levels, both of which can be detrimental to fish

Submersed Weeds

Noxious submersed weeds can quickly get out of control when phosphorus levels spike. Large communities of weeds like hydrilla and Eurasian water milfoil can literally fill lakes from top to bottom and drastically reduce fish habitat.

Floating Weeds

Duckweed masking a pond

Duckweed and watermeal are two of the smallest floating weeds, however, with high nutrient levels these weeds can completely cover a pond. With little to no sunlight entering a pond, preventing plants from producing oxygen, fish are at risk from oxygen depletion.

Alternative

To keep your fish safe and reduce pond maintenance, use a no- or low-phosphorus fertilizer. External loading of nutrients will still naturally occur from leaves and other organics entering the pond, but you're improving the situation by not contributing to the load.

References

• City of Lincoln: No-Phosphorus Fertilizer
• The Lake Experts: Nutrients and Aeration