You can forget about solar power as a constant power supply, most batteries won't make it through the night. Or you need quite a large installation that will exceed in installing costs and takes decades to break even.
Koi are very beautiful fish, but they are real carp and can grow up to 4 feet and over 30 pounds in weight. They can outlive a human in age.

This also makes them extremely hungry and actually rather dirty fish, aquatic plants are about impossible to keep with koi, they will eat and or destroy them. This all requires a massive and well-maintained filtering system to keep the water quality pristine. Next to this, because koi are cultivars they are a tad more susceptible to diseases etc. compared to their natural living counterparts. Cultivars have a significantly less natural resistance to infections because they come from well-maintained farms. Buying and keeping koi is a rather expensive hobby, especially when there are health problems, you would need a vet.
With your planned dimensions I would rather go with Goldfish, also a carp species but smaller and less demanding... These also come in many fancy varieties and also can live up to 30 years old and grow up to 40 cm in size.
You might want to read in Fred's new pond thread... Unfortunately, Fred is no longer among us. But he was a very knowledgeable Koi keeper. His thread contains loads of valuable information about it.
hi my name is fred. i am having a pond built this winter. i already have a pond. but its just basic. i plan on having a 6 foot deep pond lower pond and a raised pond above. the dimensions of the upper pond will be 8 feet width. length 5 feet.and 3 foot deep with plants in.and then water fall...
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