A decorative pot of water garden plants on your patio or deck is not only beautiful, it might just be the lowest maintenance gardening you will ever do. You can't over water them and it is very hard to under water them, too.
From one gallon to 20 or even to 500 gallons, any tub, pot, barrel, or container that can hold water can be used. The larger it is, the easier it will be to maintain healthy fish, if you desire them. Pumps and fountains are not required, but do add to the pleasure of a patio water garden and can help in maintaining fish.
Since water weighs eight and a third pounds per gallon, a 20 gallon container could weigh over 200 pounds when the water, plants, soil, and container are all added up. Select a location for the water garden that can hold the weight. A tall deck may not be able to hold a large container. If possible, place it where it can be seen from the indoors as well as the patio. If you add a fountain, the water garden container will become a focal point for the patio. Fountains do need electricity, so locations near outlets help shorten the length of necessary cords.
You will often see recommendations that water garden containers be placed in full sun conditions. If you don't have full sun, don't worry, there are plenty of aquatic plants that will do fine in a shaded container water garden, although many water lilies bloom better with more hours of sunshine than less. Full sun conditions can help to promote algae and warm water temperatures too high for goldfish. Three to five hours of direct sunlight is all that is necessary for most aquatic plants and protection from afternoon sun is often beneficial when they are grown in a pot.
Your water garden location should have easy access to the water supply. Containers with fountains will lose water more rapidly than ones without them. Keep in mind that water splashing out of fountain containers may allow algae to grow on paving surfaces, which could then become slippery.