How to Change Aquarium Water Without Killing Your Fish?

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You already know by now that regular water changes are vital when maintaining any type of aquarium setup.

Water changes are necessary for nano and normal-sized tanks, regardless of the fish species you’re housing or how powerful the filtration system is.

Water changes, oxygenate the environment, remove floating particles, and dilute excess nitrates that could become toxic to your fish, among other things.

But what happens when your fish start dying because of it?

Fortunately, I’m here to explain things.

So, let’s see why some fish can experience, you know, death during or after water changes.

Why Are Your Fish Dying After Water Change?

It’s rarely that fish die during water change unless the circumstances are really bad.

Normally, fish start dying soon after performing the water change, and there are several reasons for that.

We include here:

Water Temperature Shifts

All tank fish have specific temperature ranges where they feel comfortable. For most, the ideal temperature falls in the 72-82 range when talking about tropical freshwater fish.

Others will go as low as 65 °F and feel comfortable around 68-74 °F, as is the case with the classical goldfish.

In this context, water changes are always opportunities for dramatic temperature shifts. Especially when changing a lot more water than you should.

The official recommended water volume to change is 10-20% of the total, but not all people stick to it.

Changing too much of the fish water at once and not measuring the water’s temperature before adding it to the tank can backfire fast.

Fish will showcase signs of stress if the temperature isn’t optimal or even experience temperature shock, which can kill them nearly instantly.

You can avoid this problem by monitoring the water temperature and maybe increasing it a bit before adding it to the tank. When you do add it, do it slowly, so as not to disturb the fish in the process.

High Chlorine Concentration

Chlorine is a legitimate fish killer, with many aquarists not even realizing the danger.

Novice aquarists are the most vulnerable when it comes to poisoning their fish with chlorine which is highly toxic to them. The main source? Tap water.

Tap water is the preferred type when performing water changes which is understandable given its widespread availability.

The problem is that tap water contains lethal doses of chlorine – a chemical designed to sterilize the tap water and make it drinkable for us humans.

So, you need to dechlorinate the water before using it.

You have several options in this sense:

  • Let it breathe – Chlorine dissipates naturally from the water. Just place the water in an open, wide-surfaced container and give it some breathing space for at least 24 hours. The chlorine should evaporate completely within that timeframe.
  • Boil it – The boiling process forces the chlorine to evaporate faster, usually within less than 30 minutes. Don’t worry, boiling the water won’t remove oxygen molecules or destroy water minerals, as you may have read online. Just remember to give the water time to cool off, or you’ll have bigger problems than chlorine.
  • UV exposure – UV light neutralizes chlorine. You either leave the water container under direct sunlight or use an artificial UV light source to speed things up. Make sure you measure chlorine levels constantly to see how the dichlorination process goes.
  • Get a water dechlorinator – Yes, water dechlorinators are available pretty much everywhere. These products are known to neutralize and clean the water of chlorine, chloramine, and dangerous heavy metals that could impact your fish’s health.

Always check the chlorine content in the water after any dechlorinating technique. Remember, this substance is deadly even in small doses.

pH Level Difference

The tank’s pH levels should remain stable to support your fish’s physiological functioning.

Each fish species demands specific pH levels and can experience health problems if they drop or increase above the safe threshold.

This usually happens after a massive water change, as the procedure dilutes the available nitrates beyond the same limit, sending fish into osmotic shock.

In other words, fish won’t be able to regulate the amount of minerals and water that they can absorb from their environment.

This can cause osmotic shock, which is deadly in most cases.

Too Much Water Change

You should only change up to 20% of the tank water in one session. Anything above that can cause more problems than it can fix.

The main problem is that the tank water contains a variety of minerals and beneficial bacteria that play a critical role in the chemical stability of the ecosystem.

Changing too much of the water can imbalance the environment and cause drastic chemical fluctuations.

One of them is the ammonia boost that results from eradicating many of the colonies of beneficial bacteria.

We’ve just discussed the other problem: diluting water minerals and causing fish to experience osmosis stress and shock.

Most fish tanks only require moderate water changes, especially if you have a stable cleaning routine in place and your tank is cleaner and healthier overall.

Checking Water Parameters

You should always monitor water parameters before, during, and after every water change.

In this sense, you should keep an eye on the water temperature, pH, hardness, dissolved oxygen, the presence of ammonia and chlorine, etc.

This allows you to not only prevent but also identify problems in time so that you can handle them before further damages take place.

How to Change Water in Fish Tank?

Fortunately, changing your fish’s water is easier than it sounds.

Consider the following steps:

  • Get the tools ready – You need 2 buckets (one to pour used tank water and the other containing the freshwater), a pair of sponges, and a substrate siphon.
  • Clean the tank walls – Use the sponge(s) to eliminate all muck, dirt, or algae deposits present on the interior of the tank walls. Some residues will come off with the sponge, while others will sink to the substrate. This is where the next cleaning step comes in.
  • Vacuum the substrate – You shouldn’t be too thorough about it, not to disturb the fish and plants. But a bit of substrate vacuuming is necessary to keep the aquarium clean and fresh.
  • Prepare the water – Dechlorinate the water you plan on using and use a water conditioner for a boost in minerals and vitamins. Check the water’s temperature, pH, and overall chemical profile to make sure it’s safe for your aquatic life.
  • Change the water – You can use one of the buckets to transfer the water during the change. Remember, don’t change more than 20% of the total water volume in one go. Then you add the new batch of water, careful not to splash around or disturb the fish, plants, or substrate. Keep the hose along the tank’s walls to allow the water to flow gently along the walls. The impact will be minimal.

After you’ve completed the water change, keep an eye on your fish for the following 24 hours and monitor water parameters in the meantime.

If everything checks, you’re good to go.


Every water change contributes to the well-being and stability of the system. You don’t need to go overboard with it.

One partial water change every 5-7 days should be enough to keep the tank water clean and well-oxygenated.

Do everything right, and your fish won’t have any reason to die in the process.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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