I’m building a Koi pond that will be 9’x16’x5’ deep. All of the contractors are booked pretty solid so I guess I’m doing it myself. I have the hole dug and put a 12” trench around the bottom perimeter, will this be OK? It will also have 6 12”x 5’ concrete pillars on the corners and the middle of the long length to support a pergola Over the pond. My questions are #1 is the trench wide enough and #2 should I invest in a self leveling rotary laser to insure it’s perfectly flat a level.
What is your trench intended to do?
This is a relatively small but quite deep pond. I'm assuming you are in a very cold climate and are making your pond deep so it will only freeze on top. Putting a roof over the pond (which sounds like what you are proposing) will make growing pond plants difficult. Lilies need full sun and won't grow in full or partial shade. To keep your pond healthy, you need to have good filtration, and a good ratio of fish to plants to keep a happy eco system. If you have a good water garden nursery near by, to talk to them about your plans. My pond is about 2000 gallons, but only about 2 feet deep. 5 feet makes placing plants very difficult (shade excluded).
A rotary laser for leveling sounds good investment for non-professionals.
There are some details you might include, such as the purpose of this trench. Are you planning to pour a footing for concrete or masonry sides? If you are using a rubber liner the trench is completely unnecessary. If you don't want to invest in a laser level you can use a water level. I did this for a pond I built and it worked just fine. If you are making concrete walls I'd hire a concrete contractor and let that person deal with the details.
Are you going to set a pond pump? Is it really that necessary? I'm asking, cause i'm gonna build a pond at my backyard (not as big as yours), and a friend of mine told me, I need to install a water pump, so water in the pond would be "fresh". Is he joking or not?
I'm currently reading this article: https://www.backyardstyle.com/best-pond-pump/ please tell if you guys have tested water pump, cause for me they are quite pricy.