Chloramine
Chloramine is a chemical compound formed by combining chlorine with ammonia, used by many municipal water treatment facilities as a longer-lasting alternative to chlorine alone for disinfecting drinking water. While effective for its intended purpose, chloramine is toxic to fish, invertebrates, and the beneficial bacteria that drive a reef tank’s nitrogen cycle, making proper removal essential before any tap water is used in an aquarium.
Because chloramine is more chemically stable than chlorine, it also tends to be more resistant to simple removal methods, requiring more robust filtration than some hobbyists assume.
Why It’s Important
- Toxic to fish, invertebrates, and beneficial bacteria if not properly removed from source water.
- More chemically stable and harder to remove than chlorine alone.
- Increasingly common in municipal water supplies due to its longer-lasting disinfectant properties.
- Requires proper filtration before any tap water is used for water changes or top-off.
How It Works
Municipal water treatment plants combine chlorine with ammonia to create a disinfectant that persists longer in the water distribution system than chlorine alone. Standard basic carbon filtration is less effective at removing chloramine than it is at removing plain chlorine, which is why dedicated reverse osmosis and deionization systems typically include catalytic carbon stages specifically designed to break chloramine apart.
Breaking chloramine apart also releases the ammonia it was bound to, which then needs to be addressed as well, making a properly designed multi-stage filtration system important rather than relying on a single filtration step.
Chloramine and Related Parameters
Chloramine is closely tied to reverse osmosis and RODI water, the filtration processes typically used to remove it, and to source water more broadly, since municipal supply is where chloramine originates. It’s also related to ammonia and chlorine, both directly involved in its chemistry and removal.
Many municipalities have shifted from chlorine to chloramine specifically because it doesn’t dissipate as quickly, which helps maintain disinfection throughout a larger distribution system but also makes it more persistent once that water reaches a home. Reef keepers relying on tap water rather than a dedicated RO/DI system should check with their local water utility to confirm whether chloramine, rather than chlorine, is in use, since this affects which dechlorinating product or filtration setup will actually be effective at fully neutralizing it before the water is safe for aquarium use.
Common Misconception
A common misconception is that a standard carbon filter reliably removes chloramine the same way it removes chlorine. In reality, dedicated RO/DI systems with catalytic carbon stages are typically needed, since chloramine resists simple carbon filtration more than chlorine does, leaving it in the water longer than expected without proper equipment.
The Bottom Line
Chloramine is a stable, chlorine-ammonia disinfectant used in many municipal water supplies that’s toxic to reef life and requires proper filtration to remove. Using a dedicated RO/DI system with the right carbon stages ensures source water is genuinely safe before it enters the tank.
