Liquid Fertilizers are Fertilizers that are directly added to the liquid in an aquarium. Aquatic Plants which are free floating draw their nutrients from the water.
Adding fertilizers in the form of pellets or mixing it with the substrate will not reach these plants.
Hence, the fertilizers have to be added directly to the water. These special fertilizers contain micro-nutrients for the plants to absorb. Some Fertilizers also contain potassium.
These Fertilizers never contain phosphates or nitrates as this can result in an algae bloom. Nitrates can also harm the fish.
Never add household gardening fertilizers. The high nitrate and ammonia in these fertilizers can kill fish.