I have built 3 Koi ponds 1 for myself and 2 for others all near 10,000 gals. There is much to consider when looking into a backyard pond. What is its main purpose meaning is it a Water garden , a Goldfish pond or a Koi pond or purhaps you just enjoy a larger than normal mosquito population! They are all three different and require different features and equiptment If fish are to be part of a water garden, well they tend to eat the plants especially koi so purhaps a 2 level pond one for fish and one for plants. If only plants are your goal no filtration is needed but if it is an outdoor fish tank, than filtration is needed. Koi require 250 gals of water per Adult fish and they grow up fast! some over 24 inches Goldfish are more forgiving and can be much more densly populated. Fish ponds also usually need a UV clarifier or you won't be seeing your fish for the algae pee soup that is sure to follow. the depth of the pond is important also in northern climates it must be deep enough for fish to lay on bottom in winter and go dormant without freezing to death usually 3 feet or more. Also a hole must reamin open in pond surface for gas exchange or fish will sufficate. And last but not least a bottom drain if fish are part of the dream pond as detrious material building up on bottom of pond leads to disease and deadly gas eruption (hydrogen sulfide) and there is much more to consider. I also like a drain in bottom of a water garden as it makes the yearly clean out a snap if the lay of the land allows for it. So here is an excellent web based forum just like this one that can answer all your question
www.Koivet.com same exact community features as this site. Good luck and ask me if you have any question
l8r Gemm... Jim