Question

Is an aquarium a self sustaining system?

Answer 1

self-sustaining aquarium is one where all the inhabitants can survive without the constant care of a person. Nothing goes in; nothing goes out. Ideally the only requirements from the outside would be light and the addition of extra water on occasion.

We know it is nearly an ideal case. We can make self sustaining environment but it won't ever be 'self-sustaining'. You'll still need to feed it, provide a constant supply of electricity for the lights and pumps (stagnant tanks can't keep much alive for very long), and you'll need to replace some of the water on a regular schedule to account for all the biological and geological processes you just can't replicate in a tiny glass box.

Steps to done for making nearly a self sustainable aquarium
1 . Growth of algae
2 . Increase number of small plants inside