How do I start an aquarium ecosystem?

Hello friends,
I am sad to say, but I would like to downsize from my 20-gallon to my 3.5-gallon. I am looking to convert my 3.5-gallon tall, lidded tank into an aquarium ecosystem but am not sure how to best do that. I'd like something that would need a little less maintenance, but also be relatively cheap. I've read that self-sustaining aquariums are a myth, but also that they aren't and might even be easier. I am certainly still a noob at this, so I thought I'd ask y'all, the experts.

A little background:
I currently have a ton of baby mystery snails I plan to donate. I have 5 white skirt tetras, 1 sunshine guppy, 2 lyretail delta guppies, and a cory catfish. *I know the guppys are schooling fish, and they did have a school in my 20-gallon, but unfortunately, most have passed.*
Don't get me wrong, I love all of my fish, but I think I am looking for something new. The fish I do not use will be donated to a reputable local fish store.
My plan:
I would like to create an ecosystem with neon tetras, a few shrimp, the cory catfish (my personal favorite), and maybe a betta as a centerpiece (if not, that's fine), but I also don't want to overcrowd it.
Any recommendations would be greatly appreciated. Thank you in advance.

 

do saltwater and get a ato and you can easily make a self sustaining ecosystem

 

3.5 gallons is pretty small. A low maintenance/near self sustaining ecosystem type setup, is probably limited to plants, a snail, and maybe something like a few daphnia/scuds/shrimp. I don't think a 3.5 is large enough for any fish.

 

"3.5 tall" is that one of the 9x9x10" cube looking tanks? I don't think there are any common schooling fish that would work in this, or many fish in general that do well in a 9" footprint, maybe a solo beta.

Shrimp and smaller snails would work though. How "eco system" are you trying to go?

 

Hello friends,
. . . I would like to create an ecosystem with neon tetras, a few shrimp, the cory catfish (my personal favorite), and maybe a betta as a centerpiece (if not, that's fine), but I also don't want to overcrowd it.

Any recommendations would be greatly appreciated. Thank you in advance.

Since you don't want to overcrowd it, which is commendable, eliminate everything from your list except the shrimp.