I built a 4000 gallon pond 3 years ago. It took me 2 years to finish. I gained a lot of experience you could say. If I could do it again first I would make a much smaller above ground pond. 10 times easier to make and no digging! Digging takes a lot longer than you expect. The sides double as a seat and it’s easier to get closer to the water to see. You could make the wall about 18" tall out of brick, be sure to use a liner. For the filtration extend one side about a foot to house your filter and it doubles a seat. 3 foot is too deep and you will have trouble growing plants. Shallower is better. I would go 6x3x18". Also smaller fish are better. The large koi will out grow it and eat all the plants, comets are a much better choice, maybe even some community fish. That’s about 200 gallons total; I would go for a turn over every 2 hours. For filtration you could use an Eheim to filter it, a 2236 or 2215 would work perfect. Again saves time making a filter. Light is free and I do recommend making it out in the open not under trees. The plants will grow better and you won’t have to clean the leaves out of it all the time. Maybe add some fertilizer every once in a while and you’ll have a beautiful water garden!
http://www.aquaticplantcentral.com/f...intensive.html