So, you've gone out and bought a big water container. What's next?
Consider the spot where you want to locate your pond. Is it level? If you are placing on a concrete patio, as mine are, it is sure to NOT be level.
Think about concrete slabs and rainwater. A properly installed patio, driveway, porch, etc - will drain AWAY from any foundations. So the slab has a slope. If you are placing your pond so that the long side is with the slope, and you fill it with water, your tank will look like the water is tilted. The water will rest higher on the low side.
To fix this on my patio, I just picked up a couple of bags of play sand. I wasn't terribly concerned about it being perfectly level, so I was able to dump a pile of sand down, move the empty stock tank back and forth over it until it looked 'good' and then started filling the tank.
If a sloping line of water inside the tank will make your design sense itch, take more time on this step. Bring out the level and work the sand until the level tells you the setup is square. You will want to watch that the bottom of the tank has even support. You might get a level reading by simply tucking a paver under the low side, but the force of all that water will stress your tank if the bottom is not supported. Add the sand.
Now for the stuff that you have to use common sense about. My hobby ponds are using regular extension cords, and interior rated aquarium equipment. This is not recommended by any of the manufacturers for good reason. Most outdoor environments are wet. Traffic over the cords will cause damage. Electricity and water Do Not Mix.
Here is a multi-headed extension cord. It's currently running two aquarium filters but when I finish the setup it will also have a two-outlet air pump running as well.
Note that it is located on a brick. Here in Arizona, I don't have a lot of standing water on my property. Back in Michigan, where water might stand on my driveway for hours, my plug stand was a 8x8x8 CMU block. Bricks and blocks, whichever height you determine you need, can be recycle/salvage or purchased at your neighborhood big box store.
The cord should be placed where it won't get in the way of traffic. Slip it behind pots to keep it tight against buildings and off any well traveled pathway. If there is an area where it crosses a path, use a cord cover to protect the cord from wear. You can find these online, or you can order them through an office supply store.
Here we have my low-budget 'lid' for my cord system. This is a planter that I did not punch the drainage holes on, letting me use it as a roof for the brick and plug assembly. It's not the most beautiful solution, but it works. Use your imagination as you shop. Any pot that doesn't have a drain hole can be used in this way, given that it is large enough to cover the plug comfortably. You don't want to bend or otherwise force the cords under the cover, it should set down easily and sit stable on the ground. With a heavier cover, or if I am putting a lot of weight on it, I will shim the four sides so that the cord doesn't get pinched.
When you are happy with the placement of the cord and cover, you can top it off with a planted pot and give it a decorative function.
Stock tanks rust. They are galvanized to prolong their life, but they rust. One thing I found with my first tank is that plastic will react with the galvanized coating and compromise it. Clay, however, is inert and doesn't damage the tank. So if you plant your water plants in plastic pots, be sure to get a clay saucer to put under them. Your tank will thank you.
This shot is from a few weeks ago. I was pleased that my fish in the outdoor tanks overwintered just fine. The plants are just coming out of dormancy. I have hardy water lilies in my setups. For the winter they produce a set of very short leaves that sit an inch or so above the roots. When spring comes, they start sending out the more typical long-stemmed leaves to float on the water. You can see both types of leaves in this photo. The fish are gambusias. Even a simple minnow fish like this one can add a lot of life to a little pond.
Here is the setup in its new location. Bailey the dog approves.
Disclaimer: This is not intended to be taken as professional advice by any way, shape, or means. This is a simple description of what I have used and what has been successful for me.
Any potential pond owner that has children or dogs must take necessary precautions to ensure that the elements of any setup they use minimizes the dangers that are lurking in this hobby.
I shouldn't even have to say that if you have children that are young enough to fall into a setup like this that the setup should be treated like a full pool. It should be fenced or it should be located in an area that young children - or even water-loving small dogs - do not have access to while unattended.