Koi are voracious eaters, and can grow to be 10lbs. in two seasons. I built this ~10ft. round Pond for 5 Koi, and within 3 years I had Koi to give away. Koi love shade, and a submersed plastic end-table with the legs cut down to 6in. will give them a place to rest, while providing plenty of water overhead to swim.
FOUNDATION: In places where the winter frost penetrates the ground, and you are building a HOUSE, you need to have a foundation for it that extends below the frost-line. That way, the ground may expand and contract against the exterior walls without damaging the house. THIS IS NOT A HOUSE! ! ! Its OK that your Pond rises and falls with the Patio. In the first picture you will notice I tarred my Fountain. Its foundation goes below the frost-line, and the frozen pond needs to slide up & down the base column of the Fountain.
Dig a foundation suitable to lay your brick pond-wall to. It doesn't have to be deep or thick - perhaps 8in. wide, and 4 - 6in. deep. You don't even need forms...just dig and pour. "But" you do need to level it. Lay-up your wall, striking the joints on the face-side, but leaving the side facing to Pond rough. You will be pouring Cement against it, and the Cement/Brick should bond.
You are going to need to reinforce the Pond Walls against freezing and break-up. Of course, the best treatment for that is to place a few clear 2-liter bottles in the Pond (with a cupful of water, and capped)so that the Cap faces down. The Fish are going to need that for light in the winter too. I drove some 1/2" re-bars into the ground, and tied 3 courses of 1/2" bars to them, evenly spaced, around the pond. Mix & Place cement an "rod" it - use a stick, poking the cement down to fill every nook & cranny. When the cement stops sinking, you probably have rodded it enough. Trowel the cement off even with the top of the Bricks. You should let the cement cure for at least 3 days before stripping the forms off.
Having a rough top, you may lay brick as I show in the photo above, mortar between them, and strike the joints...if you want as Brick Top for your Pond. I went a step farther and Formed & Poured a Concrete Cap for mine. It's "fielder's choice."
The last step is to place Cement in the bottom, and trowel it. Make sure all of it slopes to the drain. Personally, I ran a 2in. PVC drain flowing by gravity into a Tank in my Basement. I filtered the water, and my Sump Pump in the Tank recirculated the water to my fountain. I tee'd the line, and with valves, was able to drain the Pond to the City's Storm Drain from my Basement. Other means of recirculating the water can be found by visiting Plumbing supply Stores, magazines, or on-line.