I know we have wise and legendary koi keepers in this forum....That being said we also have many koi keepers here that keep great koi in very good condition without spending thousands of dollars. This is a great deabte for the NEWBIE wanting to build a pond.
I think the minimum should first be defined as to where you live. Minimum means different requirements pending on location (as most of you know). Since I am a Metro-Detroit koi keeper my requirements will be a bit different.
For my 3,500 gallon pond stocked with way too many koi (ranging in size between 10 to 20 inches).
Here in Michigan I believe you need between 5' to 6' feet of depth for a koi pond. (I have 7' at my deepest point-THE COLD SUCKS for koi keeping!)
For a 3,000 gallon pond you need an aerated bottom drain and a skimmer. The pond must have PROPER SLOPE and DESIGN to be effective for the bottom drain and skimmer. Anything over 3,000 needs two bottom drains. ( I have little over 3,500 galloons and one bottom drain and skimmer)
I think everyone here would agree a Bottom drain and a skimmer are key components.
The other factor which was brought up was using concrete over EPDM.
Personally speaking I think a STRONG EPDM works fine as long as you install it properly.
Filtration is the BIG CONFUSING part of the question. As most of you would probably agree most flirtation systems have different results. Since my pond is overstocked I currently use two additional external filters for the warmer months. One of the systems is a BASIC but effective Baki shower system. The shower is connected to an external chamber stocked with media and has ample amounts of air flowing through it. Did not cost too much to build ($500 MAX). From my bottom drain I use a vortex to a pressure filter.
So I believe we must add a pre-filter to a secondary filter for proper minimum filtration. The last requirement is the upkeep of regular water changes for the minimum 3,000 gallon pond requirements.
So here is my minimum 3,000 gallon koi pond for $3,000 (With 7 Large koi, NOT included in price, LOL).
1) STRONG EPDM liner
2) Aerated Bottom Drain (4” Pipes all the way to pre-filter)
3) Vortex or similar Pre-filter to Pressure filter
4) GOOD-External Pump
5) Between 4’ to 6’ of pond depth (Pending on where you live)
6) Raised SOLID DESIGN edge (At least 20” above grade)
7) Cheap labor or a considerable amount of elbow grease on your part.
8) U.V. light
9) Weekly water changes
10) Generator
11) The will to dedicate time for koi knowledge, koi exploration and koi experience.
*-#11 is where failure and success meet.
What some people in here forget (ESPECIALLY OUR WEALTHY KOI KEEPERS) is the intimidating expense most newbies are faced with. It has been my experience that one DOES NOT NEED THOUSAND OF DOLLARS TO BUILD AN EFFECTIVE SYSTEM. The most effective tools a newbie could ask for is knowledge and patience. Theses tools are also one of the "HALLMARKS" of koi keeping and should be added to the minimum of koi keeping. Even a GREAT SYSTEM will fail with an IMBECILE as the CAPTAIN!
This is what I think a minimum filtration koi pond design should be for a 3,000 gallon pond stocked with (7-10) failrly large koi. Can you create a good system without going over $3,000? I think I could
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