Picking the best container for a patio water garden depends what kind of pot would match your garden decor and what kind of plants and fish you want to add. The requirement that it has to be water tight is only partially true. A small leak means you will have to add water more often, which can be a good maintenance tool as it keeps the water fresh. Plastic liners can be added to wooden half-barrels and waterproof coatings can be painted or sprayed into other pots to make them hold water.
It doesn't have to look like a flower pot to be used as a water garden container. Old bath tubs, galvanized wash tubs, horse water troughs, kid swimming pools, old rubber boots, and other household items have all been turned into water gardens. Use your imagination.
If you want to use a terra cotta or ceramic flower pot that already has a drain hole, you will need to try to plug it with a rubber or cork stopper. Try coating the plug with silicone sealant to help fill the gaps and to create a good watertight seal.
Wooden half barrels can be bought fresh or they may have held whiskey or wine depending on the region of the country you are in. Sometimes they have been charred on the inside to aid in the filtration of the liquid contents. It is easy to drop a pre-formed plastic liner into the wooden barrel to ensure it is water tight and free from impurities. Some hard plastic liners do not fit all barrels and they can collapse into the barrel and crack, when the weight of the water is added to it. Be sure to get a properly fitted plastic liner that sits flat on the bottom before the water is added.
Fill a fresh wooden barrel with water and see if it leaks. Small leaks will often plug themselves, as water saturates the wood and it swells to seal the cracks. Painting the interior of a barrel can aid in waterproofing. Use marine grade paint. If you purchase an old used barrel, make sure the metal hoops still hold the wood slats in place and that the barrel sits level.
Inexpensive plastic sheeting can be used to line a barrel that leaks too much, but it usually must be replaced every season and can be easily damaged when moving the pump or plants around. Lay the sheet into the barrel and add water, smoothing out wrinkles as you go. When the water is near the top, you can staple the plastic to the wood, then trim off the excess plastic. Heavy duty pond liner can be a longer lasting alternative to plastic sheeting because it is designed to hold water and will not decay in the ultraviolet light from the sun.
Darker-colored interiors seem to work better on water garden containers, because they reflect less light back into the water and visually appear to be deeper. Less reflected light means less light is available for algae to grow. Any algae that does grow is harder to see on the darker colored surfaces. Darker pots absorb more heat, which is good in the spring to speed plant growth and may be bad in the summer if it heats the water too much for fish.
Aquatic plants can be installed directly into smaller containers, while larger containers can hold several plants still in their nursery pots. The only rule for pot size is to make it large enough to look proportionate with the plants that are going in it. If tall plants are being used, consider the wind and make sure the pot won't be tipped over. Water garden containers can be as small as table top or the size of huge caldrons. The pot can even be filled with decorative gravel.
The container does not have to be visible. It can be buried in the ground or hidden behind decking. If it is buried in the ground, it can have a fountain or waterfall pump circulating the water to create more interest as water shoots up from the gravel or cascades down the rocks. This type of installation creates a very natural bird bath.