The correct option is 1 and 3 only.
Explanation
Ecosystems are broadly classified into natural ecosystems (e.g., ponds, forests) which are self-regulating, and artificial or anthropogenic ecosystems (e.g., aquariums, crop fields, gardens) which are created and maintained by humans.
Statement 1 is Correct: An aquarium is an artificial aquatic ecosystem. Unlike natural water bodies, it typically lacks a complete and balanced community of decomposers to fully recycle waste materials. Consequently, toxic byproducts accumulate, necessitating periodic cleaning and water replacement by humans to sustain the organisms.
Statement 2 is Incorrect: An aquarium is not a self-sustaining ecosystem. It lacks the inherent homeostatic mechanisms found in nature (such as complete nutrient cycling and self-regulation of populations) and relies on external human intervention for food supply, oxygenation, and waste removal.
Statement 3 is Correct: Gardens are classified as human-made (anthropogenic) ecosystems. While they contain biotic (plants, insects) and abiotic components, their structure, species composition, and maintenance are determined by human activities rather than natural selection and ecological succession.
Key Takeaway: Artificial ecosystems differ from natural ecosystems as they lack self-regulatory mechanisms (homeostasis) and depend on human intervention for their sustenance and stability.