Just found this thread, thought it was the perfect occasion to make my first post.
I've been making container ponds for the past 6 years, sometimes with a cheap mechanical filter, most often not. The smallest I've run was a 6 gallon ceramic bowl that had nothing more than a few snails, parrot's feather, water lettuce and water hyacinth that completely covered the water's surface within about a month of setting it up. It ran great, but it was a wide shallow bowl with an insane surface area to volume ration so on hot days (I live in Calgary, Canada - hot days can be hot, cold days are even colder) I was topping up the water at least twice. Floating plants have evolved in the least water stressed environment on Earth so they respirate as well as a commercial humidifier, I find that a mature bed of floating plants can dramatically increase water loss from straight evaporation off an uncovered container of water.
Over the years I've used plastic pots large enough to pot a large indoor Ficus tree, moulded resin pots about half as big, up to my favourite container: a 120 gallon glazed irridescent green/blue ceramic pot that weighs nearly 300 pounds. That one sits on a concrete pad and was sealed with silicone on the inside after the first year because it was bleeding through tiny flaws in the glazing, leaving really nasty scale on the outside of the pot (R/O water would be great, but the volume you need to replace in mid July makes that completely impractical)...
There is an artificially constructed wetland in a natural area on the edge of the city near my old house that treats storm water runoff through a series of gravel beds, water falls, marsh beds and slow moving dirt bottomed streams. Every spring I go to one of the marsh beds near the outflow of the wetland and stir up a bunch of crud from the muck at the bottom and fill 2 or 3 two litre pop bottles with water. There is also a healthy population of some species of very large aquatic snail there so if I can snag 5 or 6 I do that too. After de-chlorinating the water from the hose in the containers, I add a healthy amount of wetland water to kick-off the microbial life. Then in goes the plants. In the deeper pots I use cinder blocks purchased from Home Depot to create shelfs of different heights so that I can place marginal plants like grasses, peace lillies, mini-bullrush, horsetail or whatever else the garden centre has that year that's cool - the top of the pot needs to be an inch to half inch below the water.
I always do at least some floating plants as well, though I've learned that if you're going to buy water hyacinth, water lettuce or any other kind of leafy floating plant, save your money and buy one or two max. They spread like weeds via floating rhizomes, so even with one initial parent plant you'll likely be culling out a thick mat of them before mid summer (excellent nutrient export). I've heard of people trying to put them in a brightly lit refugium over the winter here in Calgary, though I'm not sure if it's that they need wind or a lot of UV light to maintain a healthy waxy cuticle, but water lettuce and water hyacinth almost never lasts through the 9 months of winter indoors in Calgary no matter what people try and do for them.
*I would advise for people to do their research on water lilies very thoroughly before the buy any for a container pond. Most hardy water lilies that you will find in garden centres in the spring have evolved/been bred for true ponds or lakes, with mature leaf spreads of 12-14 feet and single adult leaves which can be half as big as most people's entire container pond. There are some miniature varieties that bloom and grow more reliably in the confined space of a container, a few of the 'changeable' types (they start out one colour and over the course of a few days fade to another) that do really well in larger containers. Also keep in mind that by mid season water lilies are aphid magnets and in the confined space of a container pond they can do far more damage than they would in an actual pond, though knocking them off in to the water is an excellent food source for your fish ;). Hardy water lilies can be overwintered in a fridge if you're interested, you can google information on how to do it. If you have a plant that performs particularly well for you it might be worth it. Also keep in mind that hardy water lilies are a truly invasive pest that are decimating aquatic ecosystems that have no evolutionary history with them all across western and northern North America, so if you're going buy something that can survive a winter in your area please take care to not give it the chance to contribute to the probelm.
You can try and control temperature swings by starting your pond in the right materials - thick walled glazed ceramic containers and larger volumes help mitigate temperature swings (usually by keeping the water generally warmer night and day), down all the way to thin plastic/clear glass walled containers which will give you the wildest swings in temperature. The same things hold true for outdoor container ponds as indoor tanks - volume and stability are directly correlated, especially temperature. The colour of your container also matters. Darker containers will have hotter water in the day, lighter/reflectively glazed containers will have lower day time temperatures. Depending on what you're putting in will change which you should choose.
As for fish - I've only ever kept feeder comet goldfish in my container ponds, and they were only purchased because there was a West Nile virus outbreak here in Calgary the first year I made one of these ponds (I wanted them to eat mosquito larvae). The logic was that a) they were 39 cents and bred to be fed to larger fish anyway so if they conked it I wouldn't feel *too* bad, and b) they are cold water fish so I didn't have to worry about heating the water at night (though I have put in heaters for some of the more exotic/tropical aquatic plants you can buy). I can say that even with no filtration, water movement or artificial heat I have had fewer mortalities - and by fewer I mean none - in my pond containers than in any of my indoor tanks. In fact it was those goldfish surviving until winter the first year that necessitated me getting back in to the aquarium hobby after a decade long hiatus. Fast forward 6 years and I now have a 90 gallon reef, a 20 gallon nano reef and a 180 gallon planted community tank. Go figure.
The goldfish started spawning in my big container pond the second year I had it.
If you have fish and plants, I've found I've had the best results both in terms of plant growth, fish growth and health if there is at least some water artificial water movement. A small submersible pump to either move the water around the pond or push water up from the bottom towards the surface is all that's required. If you have fish and you don't want to do massive water changes, be prepared for the bottom of the container to get covered in a thick layer of sludge by the end of the year (especially if you have dirty, dirty goldfish), but I've found that if you have plants that come potted and anything else in the pond to provide surface area for colonizing bacteria, small water movement and a healthy population of big rooted floaters like water lettuce or water hyacinth that your nitrate levels will stay shockingly low - you won't be able to see your fish much once they cover the majority of the water's surface but at least you know they will still be there in the fall when you have to take it apart. I also barely ever fed my ponds, maybe tossing in some pellets once a week if I remembered and each summer my goldfish would still double in size (when I gave the last of them away they were all over 9 inches long and spawning almost weekly). For that reason I would recommend adding omnivores to container ponds like that so they have both any rotting vegetation and all the bugs that will be landing on the surface to try and lay eggs to eat. Parrot's feather is great because fish can nibble on the feathery leaves under the water while still giving you a show when they breach the water's surface and spill gracefully over the side of your container.
Anyway, that's my two cents. Greetings :)