caffeine thought of the day... is a perpetual aquarium possible...

[Rusty_Shackleford]

Perhaps the closest any animal hobby has come to doing this is with dart frogs.
Generally a dart frog vivarium has a false bottom to hold excess water and promote humidity. It also purposely has micro fauna in the substrate, specifically springtails and isopods. The thought is that the springtails and isopods eat and help break down organic matter such as the layer of leaf litter or frog poop on the substrate. This in turn helps make nutrients available for the plants planted in the substrate, plant respiration will also help maintain humidity. Finally the dart frogs themselves will eat the springtails and isopods. Keepers do regularly feed flightless fruit flies dusted with vitamin or mineral powders as a supplement as springs and isos are not a complete diet. Most keepers can easily go on vacation for a couple of weeks without having to worry about the welfare of their frogs in their absence.

 

[WhistlingBadger]

Perhaps the closest any animal hobby has come to doing this is with dart frogs.
Generally a dart frog vivarium has a false bottom to hold excess water and promote humidity. It also purposely has micro fauna in the substrate, specifically springtails and isopods. The thought is that the springtails and isopods eat and help break down organic matter such as the layer of leaf litter or frog poop on the substrate. This in turn helps make nutrients available for the plants planted in the substrate, plant respiration will also help maintain humidity. Finally the dart frogs themselves will eat the springtails and isopods. Keepers do regularly feed flightless fruit flies dusted with vitamin or mineral powders as a supplement as springs and isos are not a complete diet. Most keepers can easily go on vacation for a couple of weeks without having to worry about the welfare of their frogs in their absence.

It would seem realistic, in a larger vivarium, to have a self-sustaining population of insects that could sustain the frogs. The trick is finding balance, as Magnum said, as well as having enough variety to maintain good nutrition. In nature that's fairly easy, because if the animal isn't finding the nutrition it needs, it simply moves on. Tie an organism to a fixed spot, and nutrition becomes problematic. Just ask all those old west homesteaders who had to stay put and eat what they grew, or their Native counterparts who were confined to reservations. Nutritional deficiency and outright starvation were the all-too-common results.

 

[Abaddon]

I did this with a heavily planted 10gal shrimp tank - not on purpose, but after I lost my previous betta I just kind of let the tank go to seed for a while with nothing in it except plants, amanos & RCS Everything survived quite well for well over a year with no maintenance whatsoever (not even ferts), just occasional top-ups when the water level dropped. It didn't look pretty , but it seemed to establish some kind of semi-functioning ecosystem, possibly because of the very low bioload.
On another note, no one will ever convince me that amano shrimp are sensitive or delicate, lol. They were darn near indestructible

 

[GaryE]

I would take a guess and say there are several dozen harder to find fish species that could have artificial habitats with no food added. You'd need water changes for the ones I'm thinking of, but if you had grazing fish not dependent on out of water prey, I think it could work.

I had beautiful little Aphyosemion exiguum killies in a small tank where Daphnia were able to reproduce, as that species was clearly not ravenous. The tank produced a steady supply of Daphnia of all sizes for a solid six months. This was during the pandemic before I moved, and the preparation for moving and the house sale wrecked the set up. But if you took a 40 gallon, planted it and established a few live food options and a lot of plants, a tiny fish like that would work.

Now, how many people want tiny fish using large single species tanks, where the fish can be seen maybe once a week? Heterandria formosa, a few small Epiplatys and Aphyosemions, some tiny Rasbora/Bororas group fish... one fish per ten gallons in large enough tanks that they could fill in their populations...

I like feeding my fish, so I won't go there. But you can, on paper, do a lot with very large (4 or 6 foot tanks) and tiny fish, with no mistreatment of the fish. You just need enormous space.