Pond Related Questions
2-ft depth is a sensible minimum, but it is better to make the pond deeper if at all possible. The greater depth provides extra gallonage in the same area, means the temperature is more stable and provides the koi with improved exercise swimming up and down (koi in shallow ponds often take on a slightly obese "rugby ball" shape).
The exact stocking density will vary greatly depending on the filter size, frequency of water changes, amount of food given and size of the koi. The ability and experience of the koi-keeper will also have a large influence on maintaining the right water conditions. For a beginner the general rule of thumb stocking level of 8-10 koi to 2500 litres is a good starting point. This should enable good water quality to be easily maintained provided a proper filter system is installed.
Yes, other cold-water fish such as Goldfish will live happily in Koi ponds.
While not absolutely necessary, a bottom drain fitted to your pond will make maintenance a lot easier. Nowadays most people take the feed from a bottom drain straight into the settlement chamber of their pond filter rather than to the drainage system. The constant draw of water from the pond bottom into the filter helps to keep the pond clean and concentrates the solid waste in the first chamber of the filter. A smaller pipe from the settlement chamber off to waste allows for easy water changes and removal of solids.
Only if the koi will let you! Large koi generally demolish or eat most plants. Water lilies grown in large and heavy baskets that the koi cannot tip over may be alright and marginal plants will be ok provided you have a very shallow area that the koi can't get at. Most people eventually find koi and plants don't mix in the same pond.
Probably the most durable method is a concrete base with block walls, all the surfaces being rendered and then coated with fibreglass. This type of construction also allows you to contour the pond so that all the corners are rounded off and the base slopes down to the bottom drain. This allows a much better water circulation and removal of physical waste from the pond. There are no folds or creases to trap dirt as with a flexible liner and of course little chance of the pond being accidentally punctured. The main drawback to this type of construction is the relatively high cost. Many very successful koi ponds have also been built using flexible pond liners, which is considerably cheaper than the previous option and lends itself more easily to DIY construction. Before choosing the method to use it is best to read as much as possible on the various options