Liquid fertilizer

Liquid fertilizer is a concentrated nutrient solution dosed directly into aquarium water, feeding plants that absorb nutrients mainly through their leaves rather than through their roots, such as many stem plants and floating species. It typically supplies nitrogen, phosphorus, potassium and a range of trace elements like iron, in ratios formulated for planted tanks rather than for terrestrial gardening use.

Dosing usually follows a schedule, anywhere from daily to a couple of times a week, based on tank size, plant mass and how much light and CO2 the tank is running, since faster growth from more light and CO2 also raises how much fertilizer the plants can use. A dosing pump can automate this schedule in larger or more demanding setups, which helps keep dosing consistent even when a routine water change is missed or delayed by a day or two.

Because liquid fertilizer feeds the water column directly, overdosing can encourage algae just as easily as it feeds plants, so many aquarists start on the low end of a product's recommended dose and increase gradually while watching plant response. A common mistake is dosing a full recommended amount into a lightly planted tank, where there are not enough plants present to use the extra nutrients before algae does instead.