Wow, thanks for the replies!
Addy1: I will look into shubunkins. Do they get along with Koi? I’m just looking for nice things to view. Trying to create a “Zen” like escape. If the interpersonal relationships of the fish are bad, they are no fun to watch. I’ve got a small 5 gallon tank of glow fish in my office. I calculated that 5 was the magic number. But they spent all their time chasing each other and none survived. I’ve had the same three for about a year now and they swim together. Much nicer then checking each morning to see who was missing.
DrCase: I will add the skimmer. When you say to add a second water return, you mean for water to flow back into the pond or as a backup for the gravity filter incase the first gets clogged?
Dave : I saw that you were from Plymouth. Plymouth, Massachusetts is about 30 minutes from my house. I did not know you had a Plymouth in the UK. You guys really need to steal our names instead of making up new names for yourselves J
I could install 4” pipes. Given the pond is < 1500 gallons, I figured the 3 x 3 tanks would hold about 135 gallons each. It is supposed to be about 10% of the total volume, right? I could make the tanks any size. Only issue is the digging (because it is on a hill). Tendonitis is brutal.
Mgmine: I need the gravity system so the water can be used for the future hydroponic garden. How big should the boxes be? The current plan is about 135 gallons. The pond is < 1500 gallons, so I thought that was OK.
Tula, Dave : The Pond store (where I got my lining) said that 2’ is what is needed in my area. However, did you see the ;-) (wink) in my post? The pond currently slopes down to almost 3 feet. It is unlikely it will ever be inspected. If it was declared a pool, it would not be such a crisis. A pool requires a fence. My yard is fenced in anyway. Household doors going into the yard must be alarmed. We don’t have any. Worst part would be dealing with townie bureaucrats.
I’ll go down another foot.
Nepen: Yes, that is sort of like what I am doing. In my day we called it “recirculating aquaculture” but “bog” sounds more homey. I had a friend who grew tilapia in a green house. Above the fish was basil. He made more profit on the basil then the tilapia.