Can a Routine Water Change Kill Your Fish?

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We've all seen it - fish die unexpectedly after a water change. But what is the reason and how do you prevent it from happening?

If you have been an aquarium hobbyist for an extended period of time, you’ve probably had it happen – one or more of your fish die seemingly without reason following a routine water change. The routine water change is one of the most simple but also one of the most important maintenance tasks for your aquarium, but what should you do if it starts to kill your fish? In this article you will learn about the importance of water changes including what they are, when and how to do them, and some insight into some underlying problems that could be the real reason why your fish are dying off.

Why Are Water Changes Important?

If you read the feeding instructions on a container of aquarium fish food they will probably tell you to feed your fish only as much as they will consume in about 2 to 3 minutes. There are several reasons why this is a good idea. For one thing, fish food only floats for so long – if your fish don’t get around to eating it right away then it will sink to the bottom of the tank where it will join with the other accumulated detritus and debris. Second, the more you feed your fish, the more waste they will secrete and that too will add to the accumulation on the bottom of your tank.


All of that waste that collects in the substrate of your aquarium starts to break down at a certain point. With the help of certain bacteria, those wastes start to decompose in a process that produces ammonia. Ammonia is a substance that is highly toxic for fish but, as long as your tank is properly cycled, there are enough beneficial bacteria available to convert that ammonia into nitrite. Nitrite is still toxic to fish in high levels, but it is a little less dangerous than ammonia. Still, those beneficial bacteria take things one step further to convert those nitrites in nitrates. Once the ammonia has been fully converted, however, the only way to remove it from the tank is to do so literally through a water change.

In addition to removing harmful wastes and toxins from the water column, routine water changes also help to oxygenate your tank water. Just like you, aquarium fish require oxygen for respiration – they do not breathe in the same way that people do but their gills filter oxygen from the tank water. With multiple fish in your tank, the oxygen level can be depleted quickly. Adding live aquarium plants can help to boost the oxygen levels but you still need regular water changes to keep the supply fresh. Water changes are also essential after an outbreak of disease in order to remove the excess medication from the tank water once your fish no longer need it.

Why Do Water Changes Sometimes Kill Fish?

It is not uncommon in the aquarium trade for fish to die seemingly without warning. You feed your fish one final time then turn out the light and go to bed just to discover one of your favorites floating upside down on the top of the water when you wake up. It is true that water changes can be harmful to fish if you do not do them right but there is more than one reason why this might be the case. One potential problem that can occur with a water change is a sudden change in tank temperature or water chemistry. If you haven’t performed a water change in a very long time, adding fresh water to the tank could cause a sudden change in pH or water hardness – it could also change the water temperature to such a degree that your fish go into shock and die.

Another potential problem with routine water changes is that they can kill off the beneficial bacteria in your tank. Beneficial bacteria are essential to maintain the nitrogen cycle (the cycle described above in which ammonia is converted into less harmful substances) and they live primarily in the substrate, in your tank filter, and on other tank surfaces. If your water change involves using a gravel vacuum to remove a significant amount of debris from the substrate or if you also clean the filter at the same time, it could kill off a large portion of your tank’s beneficial bacteria which could induce the tank to re-cycle. Cycling is a process that can be very harsh on fish. It is also possible that the process of performing a water change causes stress to your fish and bacteria or other pathogens that are normally present in the tank take advantage of the fish in its weakened state.

The Dos and Don’ts of Aquarium Water Changes

By now it should be clear to you that routine water changes are incredibly important, even if there is a small risk that it could harm your fish. Rather than avoiding water changes altogether and subjecting your fish to poor water quality, take the time to learn how and when to perform water changes correctly. The best thing you can do is to perform small water changes on a frequent basis with a larger water change once in a while. Aim to change 5% of your tank volume twice a week or 10% to 15% on a weekly basis and use a gravel vacuum to remove accumulated debris from a different section of the tank substrate each time you do to preserve most of the beneficial bacteria. On a monthly basis, perform a larger water change of 25% to 50% of your tank volume, depending on the size of your tank.

In terms of how to go about performing a water change properly, you need to invest in a quality gravel vacuum. Most of these tools work through a siphon effect, pulling water (and any solid particles in it) up through the tube which you can then collect in a bucket and discard. When vacuuming the gravel in your tank do not forget to clean the areas under and around your tank decorations – debris tends to accumulate very heavily in these areas. Siphon a different section of the gravel with each water change to avoid killing off too many beneficial bacteria at once. When it comes to replacing the tank water you removed, be sure to match the temperature as closely as you can to the tank water and treat it with a water conditioner to remove chlorine and heavy metals before adding it to the tank.

While there are certain risks involved in changing the water in your home aquarium, it is an important thing to do and it should be done on a regular basis. To keep your fish safe, all you have to do is learn the proper way to perform a water change and then keep up with it as part of your tank maintenance routine!