Chlorine

If you’ve ever been near a swimming pool, you already know something about chlorine—it’s sharp-smelling, irritating, and clearly not something you want to be breathing in too much of. That smell comes from chlorine gas (Cl₂), a highly reactive and corrosive chemical that’s actually banned as a weapon by the Geneva Convention. So, it might come as a surprise that we intentionally add it to drinking water.

In small amounts, chlorine is harmless to humans and other large mammals, but it’s devastating to aquatic life—especially microorganisms. That’s why it’s added to municipal tap water: it kills bacteria and keeps the water safe to drink. But what’s safe for us isn’t necessarily safe for fish.

Important distinction: Chlorine (Cl₂) is not the same as chloride (Cl⁻). Chloride is a naturally occurring, stable ion that fish actually need in their water. Chlorine gas, on the other hand, is highly toxic—even at very low concentrations.

Once introduced into aquarium water, chlorine acts fast. It targets the most sensitive and exposed tissues first—namely the gills of fish and invertebrates. Because these tissues are responsible for gas exchange and are constantly in contact with the water, they’re hit the hardest. Chlorine starts reacting with organic molecules immediately, breaking down cellular structures and causing irreversible damage. By the time you see visible symptoms, it’s often already too late.

Fortunately, chlorine is easy to remove. Letting tap water sit out in an open container for 24 hours will allow the chlorine to dissipate on its own. Even more reliably, you can use a water conditioner that neutralizes chlorine instantly. Either method works—but skipping dechlorination entirely is a gamble you will lose.

Bottom line: never add untreated tap water directly to your aquarium. Even a small amount of chlorine can be lethal. When in doubt, treat the water or let it sit—your fish will thank you for it.

What About Chloramines?

Many water treatment facilities now use chloramines instead of (or alongside) chlorine. Chloramines are compounds made by combining chlorine with ammonia—yes, that ammonia. While this helps make water safer for humans over long distances (chloramines don’t break down as easily), it’s a much bigger problem for aquariums.

Unlike chlorine, chloramines do not evaporate. Letting your water sit out for a period of time won’t get rid of them, no matter how long you wait. The only reliable way to deal with chloramines is with a water conditioner specifically designed to neutralize them.

Most conditioners will break the chloramine bond, neutralize the chlorine, and convert the ammonia into a less toxic form that your beneficial bacteria can process. But here’s the catch: breaking down chloramine takes roughly five times the amount of conditioner compared to chlorine alone. That’s why it’s so important to read the label and dose appropriately—especially if you live in an area known to use chloramines (which, honestly, is most cities these days).

If you're unsure, assume chloramines are in your tap water and treat accordingly. It’s better to be cautious than to accidentally nuke your tank with something invisible.