I don't believe we can ever have a perfect balance in a tank, but we can have smooth functioning, for a while. My 120 tetra/Cory tank is smooth. I do weekly 30% water changes, clean the filters when they slow down, and occasionally rearrange a few things. The plants grow, the fish thrive.
I've had tanks for long enough to know that's a temporary state, and I'll have to intervene more for some periods. The biggest dangers are adding new fish, fish growing, and my tendency to want to try something new.
My larger tanks are the smoothest ones - the 120 and my 2 75s, which need only water changes and attention to the filters. My 40 gallon paludarium is relatively balanced, but my other 40s, and the tanks smaller than that are unstable environments needing attention.
All my tank environments only date back to my move, and this is only my third summer here. I consider these new set-ups, and they'll change with time. I'm careful with how I stock tanks, and like my dwarf Cichlids at 10-15 gallons per fish, and larger fish at 20 to 30 each. Cichlids grow, and become that very big kid at one end of the teeter totter. No balance there.
I have sunlight in the room, and that causes seasonal algae flareups. I also get Cyano sometimes, like now. I like having that light for the terrestrial plants, both roots in the tanks and filters, and potted. So I have a balance to strike there - more algae and Cyanobacter in those tanks, but a nicer room.
If the truly balanced aquarium weren't a myth, I'd probably quit the hobby. I like the adjusting, experimenting, and generally fiddling around side of the hobby. If something looks perfect, take a photo of it. It won't last.
