There’s basically 3 types of ornamental ponds — The Water Garden, The Goldfish Pond and The Koi Pond. The Water Garden which has only water and plants (and maybe a frog or two) is low-maintenance and intended to accent the beauty of the plantings only. The Goldfish Pond, also relatively low-maintenance, needs aeration, water, plants and a small filtration system. The Koi Pond requires excellent water quality. The need for adequate and external filtration is a must.