I have koi ponds 1 is 4200+ and the other in the making is 13,000+
Be prepared to spend some bucks. Just the UV alone cost well over $1200 for the 13,000+
The following is from a pro koi keeper an breeder.
How big does a koi pond need to be?
While very young koi can be kept in a relatively small container, they grow rapidly and require considerable room in which to do so. Also part of the consideration is the amount of filtration associated with the pond. We recommend an absolute minimum of 900 gallons as a koi pond. A much better size, even for a beginning hobbyist, would be 2000 to 5000 gallons. Some koi ponds are 10,000 gallons to over 200,000 gallons.
How deep should a koi pond be?
Koi can live in very shallow water, but are not comfortablewithout access to deep water. Particularly in hot climates, like Texas, they require deepwater and shade as refuge from the hot summer sun. We recommend an absoluteminimum depth of 42 inches for a koi pond. Koi derive musculature by swimmingup and down in the water and a better depth for a koi pond would be 60” to 84”.Koi ponds can and should have both deep and shallow areas.
How many koi can I keep in my pond?
Koi can grow very large, as long as three feet in some cases. While many small koi can be kept in a pond, the same pond will hold only a few large koi. The capacity of a pond is also very dependent on the type and amount of filtration the pond has. For most koi keepers, about 50 to 100 gallons per small koi (under 12”), and 500 to 1000 gallons per large koi (over 24”) is a good rule of thumb in a properly filtered pond. A good philosophy for collecting koi is to be very selective and have a smaller number of higher quality specimens.
What type and how much filtration does a koi pond need?
Koi will eat a lot; this is especially true in warm weather.This implies a great deal of wastes that must be filtered from the pond.Biological wastes come in many forms and require different approaches tofiltration in the same system (pond). We recommend that koi ponds be plumbed insuch a way as to remove water from the bottom, mid-level, and the surface forfiltration. We also recommend at least two separate filtration systemsincluding two pumps. Usually this consists of one system for bottom waterfiltration and another system for surface (skimmer) and/or mid-level waterfiltration. We do not recommend circulating any water that is not filtered. Thecombined filtration system should filter and return at least one pond volumeper hour on small ponds (under 5000 gallons) and one volume per hour and a halfon larger ponds (over 5000 gallons). A combination of mechanical (solidsremoval) and biological (ammonia conversion to nitrate) filtration is requiredto properly filter a koi pond.