Why you should not add tap water directly to your aquarium in the UK

Four months ago, I began my fishkeeping journey and chose to document it through a video series on my YouTube channel, hoping to support others who are new to the hobby or considering setting up their first aquarium. While I’m grateful for the advice I’ve received along the way, one recommendation keeps coming up—how to make routine water changes quicker and easier.

The most common suggestion is to fill the aquarium directly from the tap. However, to help beginners avoid the distress of losing their fish, I’ve written this article explaining why adding tap water straight into your aquarium in the UK is not a good idea.

Adding tap water straight into an aquarium in the UK can seriously harm or even kill your fish. The issue isn’t the water itself—it’s what’s in it and how it behaves in a tank environment.

Here’s why you should never add untreated tap water directly:

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1. Chlorine and Chloramine (Highly Toxic)

UK tap water is disinfected with chlorine or chloramine to make it safe for humans. Unfortunately, these chemicals are toxic to fish.

• They damage fish gills, making it hard to breathe

• They can kill beneficial bacteria in your filter (which keep the tank healthy)

Even small amounts can cause stress or death.

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2. Heavy Metals

Tap water may contain trace amounts of metals like:

• Copper

• Lead (especially in older pipes)

Fish are far more sensitive to these than humans, and buildup over time can be dangerous.

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3. Temperature Shock

Tap water is often a different temperature from your tank.

• Sudden changes can shock fish, weakening their immune system

• This can lead to illness or death

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4. pH and Water Chemistry Differences

Tap water chemistry varies across the UK (hard vs soft water areas).

• Sudden pH shifts can stress fish

• Sensitive species may not tolerate quick changes

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5. Harms Beneficial Bacteria

Your aquarium relies on good bacteria to break down waste (the nitrogen cycle).

• Chlorine/chloramine can wipe them out

• This can cause toxic ammonia spikes

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What You Should Do Instead

✔️ Use a water conditioner (dechlorinator)
This instantly neutralises chlorine, chloramine, and often heavy metals.

✔️ Match the temperature
Let the water sit or mix hot/cold to match tank temp.

✔️ Add water slowly
Avoid sudden changes in chemistry.

✔️ Test your water if possible
Especially for pH and hardness.

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Simple Rule

👉 Tap water is safe for humans, but not for fish unless treated first.

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