Welcome to water gardening in containers. Many of the principals about container water gardens are similar to ponds. However, there are some major advantages to container water gardens. First and foremost, they are moveable. You can put them in places you might not ordinarily consider putting a pond, such as a deck, patio or roof (assuming of course the structure can support the weight). They are easy to set up, and they are relatively inexpensive. And, especially for someone new to water gardening, they are a wonderful way to learn and experiment before you invest in a larger installation.
Now some basics about setting up your own water garden. When creating a water garden from scratch, you are in essence taking 'dead' tap water and bringing it to life, establishing an entire ecosystem of water, plants, bacteria and other critters. In order to create a healthy environment for the plants and fish, several elements are needed to keep it in balance.
The Container — depending on your environment, your imagination, and your budget, there are many possibilities. For my taste, I like working with containers that are at least 20" wide and about 20" high. Though it is possible to use smaller containers, the water tends to heat up faster, (which is tough on fish), and it can get crowded rather quickly. Also, make sure that the container will not leech any toxins into the water.
The style of container can range from the rustic whiskey barrel (preferably lined), to exotic Asian style pots, to classical Mediterranean pottery. I've also heard of bathtubs, old Jacuzzis and troughs. It just depends on what's available and what strikes your fancy.
The Plants — In assembling the plants for my container water gardens, I consider the three types of plants needed to establish a proper ecosystem and keep algae growth to a minimum: 1) oxygenators, which are fully submerged. As their name suggests, they put oxygen into the water, absorb waste and provide protection for fish. 2) Water surface covering plants (such as water lilies or hyacinths) to shade and cool the water and keep algae under control, and 3) bog or marginal plants to consume excess pond nitrogen.
In addition to the biological functions of the plants, there is of course the aesthetic. One of the key things about working with container water gardens is the size and proportion of the plants. As a rule of thumb, I choose at least one tall upright (2 to 3 feet), one bushy upright (1 to 1-1/2 feet) and one creeper. In addition, I also love including miniature water lilies in the summer and Water Hawthorne for the winter. If budget is a concern, there are several inexpensive floating plants that serve the same purpose: partially shade the water to keep it cool and algae under control. There are numerous books on the topic that provide wonderful information on the care and growing habits of many types of water plants.
Now that you have the container and plants in place, the last element is the wildlife. Some will come of their own accord such as birds and butterflies, and some will need to be imported. If you have no fountain element, you will definitely need some sort of mosquito control. The easiest is a little fish called - you guessed it, Mosquito Fish. They are incredibly tough and will do the job. Other small fish can work as well - the common goldfish, swordtails, platys, etc. Make sure you don't overcrowd your garden with fish; 2" of fish for every gallon is the recommended number.
Water snails are also needed to help digest the waste, and if you like, there's always room for small frogs, (assuming the neighbors don't mind) turtles, and even freshwater shrimp.
Though these water gardens are designed to be self-sustaining ecosystems, many folks want water in their environment so they can have the additional pleasure of listening to it. Underwater pumps are now easily available, and can be set simply in the water. Make sure that if you do get a pump that it is designed for outdoor use. Also, if there is no established outdoors electrical source consult with a qualified electrician before plugging anything in.
Most importantly — experiment, have fun, and enjoy the beauty and pleasure of a container water garden.
The following are some of the plants that I have worked with and read about that can create wonderful gardens.
LILIES AND SIMILAR
Pygmaea 'Helvola' (hardy)
Joanne Pring' (hardy)
'Colorato' larger leaves (tropical)
Water Hawthorne (winter)
BOG OR MARGINAL PLANTS
Japanese Sweet Flag
Rush - Common or spiral