About Chloramine

If your water comes from a municipal supply, there’s a good chance it contains chloramine — and standard dechlorinators won’t fully handle it. Chloramine is a compound used to disinfect drinking water. Unlike chlorine, which dissipates quickly, chloramine is stable and persistent, which is why water treatment plants have increasingly adopted it. If you’re not sure whether your local water supply uses chlorine or chloramine, contact your water utility and ask.

Why chloramine is a problem for aquariums

The same chemicals that make tap water safe to drink can be highly toxic to fish and the beneficial bacteria that keep your tank stable. Chlorine is relatively simple to remove — it reacts quickly with sodium thiosulfate-based dechlorinators and is neutralized instantly. Chloramine is more complicated.

Chloramine is chlorine bonded to ammonia (NH₃). Standard dechlorinators will break that bond and neutralize the chlorine — but the ammonia is left behind, free in the water. Using a basic dechlorinator on chloramine-treated water solves one problem while creating another.

Understanding the ammonia left behind

The toxicity of the remaining ammonia depends largely on the pH of your water. Ammonia exists in two forms: the more toxic unionized form (NH₃) and the less toxic ionized form (NH₄⁺, also called ammonium). At lower pH levels, ammonia is more likely to exist as NH₄⁺. As pH rises above 7.0, the balance shifts toward the more toxic NH₃ form. The same ammonia reading can represent a very different level of risk depending on your water’s pH — saltwater tanks, which run at a higher pH, are particularly vulnerable.

How to remove chloramine

There are a few approaches, and the best one depends on your setup.

The easiest option is to use a water conditioner specifically formulated to handle chloramine — one that neutralizes both the chlorine and the ammonia together. Fritz A.C.C.R. and Fritz Complete Water Conditioner both detoxify ammonia in addition to removing chlorine, making them a good fit for chloramine-treated tap water.

If you prefer to remove the chlorine and address the ammonia separately, sodium thiosulfate handles the chlorine portion quickly and inexpensively. From there, you have a few options for the ammonia:

• A healthy, well-established biological filter may be sufficient to handle the ammonia load on its own, particularly at lower pH levels and with a moderate fish load. To test whether your filter can keep up, start with small water changes — around 5% — and increase gradually while monitoring ammonia levels closely. If you see signs of stress at any level, your filter may not be keeping up.
• If your filter can’t keep up, increasing aeration and biological filtration capacity will help. In some cases, aquarists manage ammonia by reducing fish load, upgrading to a higher flow rate filter, or adding reverse osmosis water to lower pH and reduce NH₃ toxicity.

The simplest approach for most hobbyists

For most home aquariums, the easiest and most effective option is a water conditioner that handles both chlorine and ammonia in a single step. Treat your replacement water before or as you add it to the tank — the reaction is instant, no need to mix and wait. Follow the product dosage instructions based on your tank volume.