Gravel has many benefits in a fish pond. Rocks and gravel are important for a healthy ecosystem and extend the life of your pond liner.
Gravel is placed in the bottom of a fish pond to cover the pond liner, similar to covering the bottom of a fish tank with gravel. Typically ponds owners line their ponds with large gravel and rocks.
Gravel- and rock-lined ponds protect the pond liner from UV rays which deteriorate the pond liner and add weight to prevent the pond liner from shifting. Gravel and rocks add a decorative element to fish pond setups.
Fish pond bottoms become covered with sludge that forms from fish waste, and dead and decaying material. Rocks and gravel provide a surface for bacteria to grow that will eat away the sludge and prevent it from polluting the pond water.
The bacteria that grow on the gravel are the same bacteria that grow in a pond filter. A gravel layer will make cleaning the fish pond easier since it removes much of the sludge the same way a pond filter removes toxins.
Water gardeners and koi pond keepers debate whether gravel-lined ponds are beneficial. While many people believe gravel and rock have beneficial qualities, some people believe that gravel will form a stagnant water zone at the bottom of the pond full of rotting waste, where clean, moving water cannot reach to flush it out.