Reddit mentions of Stability, 100 mL / 3.4 fl. oz.

1. I got the container at a place near Nashville called Southeastern Salvage. I couldn't find anything online, but here’s what the barcode sticker says:
   HF1562LAW
   LG antique white planter
   745896421086
   
2. Paver base from HD to level the container.
   
3. The fountain is the Beckett Container Garden Fountain Kit. It's available at Home Depot and Amazon. I added some aquarium filter media which really helps with cloudy water.
   
4. Typical pond liner from HD. This was the smallest size at my store and still way more than I needed.
   
5. Aquarium safe silicone.
   
6. River pebbles from Home Depot for substrate - I know most pond-owners don't recommend this, but my container is small enough to vacuum the gravel when it gets gross. This choice was may too natural/orangey and makes the fish harder to see.
   
7. I happened to have a broken concrete block (like this) which made a a nice base to keep the pump off the bottom and leaning another piece on it gives the fish a hiding spot.
   
8. Floating plants: water lettuce and water hyacinth. Aquatic plants: hornwort, anacharis and golden creeping jenny I pulled out of my flowerbed and tossed in.
   
   The build was pretty simple, when you break it down. It took a while just with life stuff getting the way and I didn't do everything exactly to this order - adding floating plants before I was done sealing the edges, didn't do the final liner trim until very last etc., but this is the gist:
   
   
9. I chose the location and made a pile of paver base wider than the bottom of the container.
   
10. Set the container on the paver base and set a large bubble level across its top. Pushing down, I worked the container back/forth/side/side until it was level all the way around and felt pretty stable.
    
11. Spread out the liner in the container (letting excess hang over sides) and filled with water making sure there were no bubbles or anything. I should have done a better job working out/consolidating the wrinkles/folds at this point. It was pretty hard when it was full, but it turned ok (not great). Once the container was full, I trimmed most of the excess to make it easier to work with, leaving about 12" all around.
    
12. I went ahead and put in some gravel, the concrete block pieces, and fountain at this point, though I should have waited to make it easier to work out wrinkles in the liner. I used the fountain to remove water until the water level was about 6" below the rim to keep the liner pressed against the sides, but allow me to access the edge. Then, I tucked the liner under the rim of the container and trimmed some more excess. Next, I used aquarium safe silicone to adhere the liner under the lip of the container rim so the edge of liner is concealed by the lip of the container. I used clamps in a few places to hold it in place while the silicone set. I had to be careful to keep the outside dry where the liner contacted the silicone to get a good seal. I did another pass later to remove the final bit of excess and add silicone where needed.
    
13. Filled it back up adding pond water conditioner, added plants and fish. I used this new tank bio starter stuff too. I lost 1 goldfish on day 2 so it was probably on its way out anyways but the rest seem healthy after about 2 months.

Personally i'm a Betta fan, so if you haven't ruled them out completely then here's my suggestion:

get the tank all set up except for plants and begin cycling. I suggest picking up Seachem's Prime for a dechlorinator (it's cheaper in the long run since it's more concentrated and it has some other benefits. This specific bottle treats ~2,500 gallons). You will also want to order the API master test kit if you don't have it yet.

If you want a short cycle try Seachem's Stability to quickly establish the proper cycle and/or get some filter media (the brown gunk) from a friend or LFS. Proper cycling can take weeks to months on a new tank.

Once the tank is established (or before if you're treating with stability) get some good low-light plants like anubias (these are especially good for Betta's, with their broad leaves) and java fern. Mosses are good too. Keep an eye out for pond snails which can easily overtake a tank. Driftwood or other decor can go in now too, get all your aquascaping done before you add any fish.

Now, with the tank all set up and cycled (keep that bacteria fed w/ fish food or pure ammonia) you can go and get your fish. If you are worried about aggression and still want a Betta bring along a small mirror. You can some idea about the fish's temperament, but this test is imperfect. I have a male that flares at anything that moves, but I housed him with both shrimp and Otos (my sorority ate the shrimp when I had them in there) and he just ignored them.

Even if you have an aggressive Betta getting a larger mystery or nerite snail will make that a moot point, since they have thick protective shells. Some Bettas will attack them a little, but when they get no reaction they will get bored and eventually learn to live in peace with it. (contrary to popular belief Bettas can be fantastic in a community tank, not all females are docile little angels, and not all males are ruthless killing machines. There are always some aggressive Bettas, but generally they are only aggressive towards other Bettas and fish that look similar. Note that if you have fancy male guppies then a female Betta is a better option)