Well here's this little experiment- I filled a ceramic pot with tank water and threw a handful of substrate and some plant trimmings in there, weeks ago. It has no drainage hole so attempts to put terrestrial plants in it have failed in the past, but it's such a nice pot I don't want to risk cracking it by trying to drill holes. I thought: why not a small container pond? Problem is our exterior outlet on this level of the deck doesn't work, so I have no filter running. That rules out fish. It's just over five gallons.
There's a terra cotta pot in there on its side- at first I thought a few endlers, guppies or mosquitofish could survive in here and have some shelter.... I threw in some watersprite, duckweed, subwassertang, hornwort and elodea. The watersprite and elodea outright died. Subwassertang got pulled for my fry box a few weeks ago. Duckweed is super happy. It's multiplying, has better color than in my indoor tank, undersides turn purple and roots are thicker.
Hornwort grows differently than from in my tanks. Needles are very fine, short and dense. It's rather pretty. A few days ago I cleaned out tons of dead, rotting stuff left from its transition period and now there are just a few bunches of it in here, but I'm sure it will keep going.
It's kinda scummy down in there, from the dying plant bits. I did toss in a few small malaysian trumpet snails just for the heck of it, don't know if they're alive. I keep a window screen over it most times, but don't mind gathering the few mosquito larvae that show up, to feed my tetras.
It doesn't get fed. When the water gets low, if it isn't raining I add a bit of tank water, that's it. I'd like to try water lettuce, I think it would look nice, but I haven't yet made the trip out to the lfs where I've seen it for sale.