Do You Need a Dechlorinator? Tap Water, Chlorine and Chloramine

If you fill your tank from the mains, use a dechlorinator every time, no exceptions worth risking. That is the practical answer and it holds for nearly everyone. The water company adds chlorine, or increasingly chloramine, to keep the supply safe for people to drink, and both are harmful to fish and to the bacteria your filter depends on. A small bottle of conditioner costs very little and lasts months. A wiped-out filter or a tank of gasping fish costs you a great deal more. I have kept fish on hard tap water for years and I have never once skipped it on a water change.

What a dechlorinator actually does

A water conditioner, or dechlorinator, neutralises the chlorine and chloramine in tap water so it is safe to put fish into. Most products do this with a reducing agent, commonly sodium thiosulfate, which reacts with the chlorine almost instantly and turns it into harmless chloride. Many conditioners also do a second job: they bind or detoxify the ammonia that chloramine leaves behind, and lock up traces of heavy metals like copper that can come through old pipework.

That is the whole of it. It makes tap water safe. There is one thing it very much does not do, and I will come back to it, because it is the single most common misunderstanding I see from people on hard water.

Why tap water is not safe as it comes

Chlorine and chloramine are disinfectants. Their job is to kill microorganisms, and they are good at it, which is exactly the problem in an aquarium. Chlorine damages fish gills and skin directly, and at the same time it kills the beneficial bacteria living in your filter, the colony that quietly converts fish waste into less harmful compounds. Lose that colony and you are back to an uncycled tank with ammonia climbing, which is one of the quickest ways to kill a stocked aquarium. If that cycle is still new to you, the nitrogen cycle explained and fishless cycling are worth reading first.

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The Hard Water Aquarium

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Chlorine, chloramine and why standing water is no longer enough

You may have been told to fill a bucket and leave it overnight so the chlorine gasses off. That used to be sound advice, and for plain chlorine it still works, because chlorine is unstable and evaporates from standing water, faster still if you aerate it with an airstone for a day.

Chloramine is a different animal. It is chlorine bonded to ammonia, deliberately more stable so it lasts longer in the pipes, and it does not meaningfully gas off if you leave the bucket to stand. Worse, when it does slowly break down it releases free ammonia, which is toxic in its own right. This matters more every year, because water companies are steadily switching over. In the UK, more than a tenth of the supply is now chloraminated and the figure keeps rising; Scottish Water has introduced it across areas including Aberdeenshire and around Inverness, and Edinburgh’s supply moved over too. Which disinfectant reaches your tap depends entirely on your local company, and some use chloramine only at certain times or to deal with a specific problem.

The practical takeaway is simple. I stopped keeping standing buckets years ago. A conditioner that is labelled to handle chloramine deals with both cases in seconds, and you do not have to guess what is in the water that day. If you want to know exactly what your supply contains, your water company publishes a quality report for your postcode.

How I treat water on a change day

My routine is about as simple as it gets. My tap is hard, so it goes more or less straight into the tank, with no softening and no fuss. I work out the volume of new water going in, dose the conditioner for that volume, match the temperature roughly to the tank, and refill. That is it. Getting the volume right matters for dosing, so if you are unsure how much water you are actually changing, the water change calculator and the volume calculator take the guesswork out of it.

A word on dosing: follow the label. Most conditioners are forgiving and a small amount over the stated dose does no harm, but there is no prize for drowning the tank in it either. The usual approach is to dose for the new water you are adding, or for the whole tank volume if you prefer the belt-and-braces version. The wider water-change routine, how much and how often, lives in the water changes guide.

Are there times you genuinely do not need it?

A few. If you keep fish entirely on reverse osmosis or collected rainwater, there is no chlorine to remove, although you will have remineralised that water, which is a separate job. Private well water usually has no added disinfectant, but test it rather than assume, since wells bring their own surprises. And a very large activated carbon bed can strip chlorine given enough contact time, though I would not trust it as my only line of defence.

For everyone else on mains water, the honest sums are not close. The downside of using a conditioner is a few pennies a change. The downside of skipping it on a day your company has dosed the supply harder than usual is a damaged filter and sick fish. I use it every time and I would tell anyone else to as well.

The hard-water catch: a conditioner is not a water softener

Here is the misunderstanding I promised to come back to. A dechlorinator does nothing to your water’s hardness. Nothing at all. My tap is around 22 dGH and it is still around 22 dGH after I have conditioned it. Plenty of people on hard water buy conditioner expecting their 20-plus dGH tap to soften, and it never does, because that is simply not what the product is for.

If you actually need softer water, for soft-water fish or shrimp that will not thrive on hard tap, the only reliable route is reverse osmosis water, remineralised back up to the level you want. Do you need RO water walks through whether it is worth it for you, the RO mix calculator helps you blend it, and the remineralise calculator sets your target hardness. For most hard-water keepers, though, my honest advice is the same as ever: do not try to soften your tap for the sake of it. Pick fish that love the water you have, and read hard water fish before you buy.

A couple of practical notes

Because many conditioners temporarily detoxify ammonia, they are a useful thing to have on the shelf during a fish-in cycle or an ammonia emergency, buying you time between water changes. That is a stopgap, not a fix, and the real answer is still dilution and patience; see high ammonia in the aquarium if you are in that spot. Beyond that, keep the bottle capped and out of sunlight, and do not lean on a conditioner to rescue genuinely bad water. It makes tap water safe to use. The rest of good water, stable parameters and a maintained filter, is down to you, and the water parameters guide lays out what to aim for.

Bottom line

Use a dechlorinator on every water change unless you know for certain your source has no chlorine or chloramine in it. Choose one that handles chloramine, since standing the water no longer does the job for most of us. And remember what it is for: it makes your tap safe, it does not change how hard that tap is. On hard water, that distinction saves a lot of wasted money and disappointment.