From the perspective of the water lily and the broader pond ecosystem it inhabits, the introduction of fish is a complex relationship with significant benefits and notable challenges. The overall health of the aquatic plant community is deeply intertwined with the activities of its piscine inhabitants.
Fish are a vital source of essential nutrients for water lilies. Their waste products, primarily ammonia, are processed by beneficial bacteria in the pond into nitrates. Nitrates are a preferred form of nitrogen, a crucial macronutrient that water lilies absorb through their roots to fuel vigorous leaf growth, strong stem development, and prolific flowering. In a closed system like a pond, this constant, natural fertilization can be far superior to relying solely on the often-limited nutrients found in the underlying soil. Furthermore, fish respiration releases carbon dioxide into the water, which can be absorbed by submerged plant parts and, to a lesser extent, contribute to the photosynthetic process, supporting the plant's energy production.
One of the most direct benefits fish provide to water lilies is the suppression of algae. Many fish species, such as koi and goldfish, are opportunistic feeders and will consume string algae and other filamentous forms that compete with water lilies for light and nutrients. By keeping algal blooms in check, fish help ensure that sunlight can penetrate the water's surface and reach the large, flat leaves of the water lily. Unobstructed access to sunlight is critical for photosynthesis, allowing the plant to produce the energy required to spread its rhizomes, produce new pads, and generate its iconic blooms. A clear water column, maintained in part by fish, is therefore a direct contributor to the plant's vitality.
Water lilies can fall victim to various aquatic pests, such as aphids and larvae of certain insects that chew on leaves and stems. Fish act as a natural, mobile pest control unit. They will actively consume these pests, preventing significant damage to the lily pads. This protection helps maintain the integrity of the leaves, which is essential not only for photosynthesis but also for the plant's aesthetic appeal. Healthy, undamaged leaves are better able to regulate gas exchange and remain buoyant, providing the necessary shade that helps keep water temperatures cool and further inhibits algae growth, creating a positive feedback loop for the entire pond environment.
Despite the benefits, certain fish behaviors can be detrimental. Larger, bottom-feeding fish like koi are notoriously curious and can root around in the substrate. This activity can dislodge the delicate, fleshy roots and rhizomes of the water lily, disturbing its anchorage and potentially uprooting young plants. The turbidity (cloudiness) caused by stirred-up sediment can also settle on the leaves, blocking light. Additionally, prolific fish populations produce significant waste. While this can be beneficial as fertilizer, an excess can lead to an overabundance of nutrients (eutrophication), potentially fueling the very algal blooms the fish help control, and creating an imbalance that stresses the entire plant community.
For a harmonious relationship, the pond must be designed with both plant and fish needs in mind. To protect water lily roots from foraging fish, it is advisable to plant them in sturdy, mesh-bottomed containers and cover the soil surface with a layer of large gravel or stones. This allows for nutrient exchange while providing a physical barrier. Managing the fish population is also critical; overstocking should be avoided to prevent nutrient overload. Choosing smaller, less destructive fish species, such as mosquito fish or certain types of minnows, can be a preferable option for a planted pond focused on the health of the water lilies, as they provide the benefits of pest control and fertilization without the destructive rooting behavior.
