What typically causes transparent green water in a pool?

Transparent green water in a pool is primarily caused by copper ions in the water. When copper is introduced into the pool, often from corroding copper pipes, poorly formulated algaecides, or certain types of pool equipment, it can lead to a greenish tint in the water. This issue is usually coupled with other factors like pH levels and water chemistry, but the key contributor in this instance is the presence of copper, which can impart a green color.

Algae growth, while a common cause of water discoloration, typically leads to a more opaque, green appearance rather than a transparent one. Excess chlorine can cause chloramine formation, leading to cloudy water but not necessarily a green tint. High pH levels do impact the effectiveness of chlorine and may lead to other water quality issues, but they are not directly responsible for the transparent green coloration. Thus, copper ions are the correct answer for this specific color problem.