Why is it difficult to create a balanced aquarium with plants and animals?

Creating a balanced aquarium—where plants and animals sustain each other through respiratory and metabolic exchange—is difficult to achieve in practice, particularly in large-scale or normally stocked tanks and especially in marine environments.1

While the theoretical model suggests that animals provide carbon dioxide and waste for plants, while plants provide oxygen and consume waste through photosynthesis, maintaining this equilibrium is challenging for several reasons:

• Insufficient Plant Growth: In many systems, particularly marine aquariums, plant growth is typically not robust enough to utilize all the nitrate produced by bacteria from metabolic waste.1
• Scale and Stocking Density: Achieving a true balance is rarely feasible in a normally stocked aquarium or on a large scale, as the biological demands of the animals often exceed the capacity of the plants to process waste or regulate gas levels.1
• Environmental Variables: The process relies on specific light intensities to drive photosynthesis. If light levels are inconsistent, the respiratory exchange between plants and animals can easily become disrupted, necessitating artificial filtration and aeration to maintain water quality and prevent asphyxia.12