Water gardening can be a low maintenance hobby IF your pond is balanced with the correct types and amounts of plants and fish in relation to its surface area. Following are a few basic guidelines to help you achieve such an ecologically balanced water garden.
Before we begin though, please realize that larger water gardens are generally easier to balance and maintain because the water temperature does not vary as much over a 24 hour period as it does in small ponds, thereby resulting in greater stability of the system. That means the ratios listed below are even more critical for smaller water gardens.
Remember too that a balanced pond will maintain water clarity and quality naturally, even if it does NOT have a filter system. If, however, you plan to stock more than the suggested amount of fish and/or less than the recommended amount of plants, you might need a filter to improve water conditions.
Note: For a rectangular pond, surface area equals length x width. For a circular pond, surface area equals 3.14 x radius squared.
Oxygenating or submerged plants are plants that grow completely below the surface of the water. These are the most important plants in balancing the water garden. Not only do they oxygenate the water and absorb carbon dioxide during the day, but more importantly they compete for the nutrients that the algae need to survive. They also provide an area for your fish to spawn in and to hide from predators. Examples of these types of plants are anacharis, coontail, and fanwort.
Oxygenating/submerged plants should be stocked at a rate of one bunch for each one to two square feet of pond surface area.
Floating foliage is any plant or portion of a plant that floats on the water's surface and blocks sunlight, thus preventing algae growth. Examples of these types of plants are water lilies (both hardy and tropical) and lily-like aquatics. Other floating plants such as water hyacinth, water lettuce, duckweed, and fairy moss serve double-duty by absorbing nutrients (that would otherwise fuel algae growth) through their dangling root systems.
Floating foliage should provide ~50%-80% surface coverage (~60%-70% is ideal). If your pond is in full sun, you should aim for ~80% coverage and if your pond is in shade, you can aim for ~50%-60%.
Marginal, emergent, or bog plants are aquatic plants that basically need shallow water to survive while providing some shade to the water garden. They also provide the texture and height needed to naturalize the water garden to its surroundings. Examples of such plants are lotus, cattails, iris, rushes, sedges, and arrowhead. Many of these plants are also very effective at filtering the water.
Marginal, emergent, or bog plants contribute to the previously mentioned ~50%-80% surface coverage ratio.
Fish and Snails
Goldfish - To allow for growth and reproduction, the recommended stocking rate to start with is only 1/2" of goldfish for each 1 square foot of pond surface area. The recommended maximum rate is 1" of goldfish for each 1 square foot of pond surface area.
Koi - Again, to allow for growth and reproduction, the recommended stocking rate to start with is only 1/4" of koi for each 1 square foot of pond surface area. The recommended maximum rate is 1/2" of koi for each 1 square foot of pond surface area.
Note: When feeding fish, only feed 1-2 times per day with just enough food to be eaten within a five-ten minute period. Scoop out any remaining food that is floating around, as this leftover food provides algae with the necessary nutrients it needs to survive if it is left in the pond.
Snails - The recommended stocking rate for Japanese Trapdoor and Ramshorn Snails is one snail for each one square foot of pond surface area. Be aware that the Great Pond Snail is not really a welcome addition to the pond as it can eat your plants, as well as be a host of anchor worms that can harm your fish.
Although not a stocking ratio, you should also keep in mind that if you are using a pump to assist in balancing your water garden, the entire volume of water in the pond should be circulated once every one to two hours.