Can Massive Water Changes Kill Fish?

Tokyo bitterling adult male fish
DigiPub / Getty Images

Regular partial water changes are recommended for your aquarium, but massive water changes can lead to problems. There are myths and misunderstandings about why this causes trouble.

A typical tale of woe is an aquarium owner who hadn't been doing partial water changes since he set up the tank a year ago. When a friend noticed the gravel was a little dirty, the owner learned he should have been doing frequent water changes and cleaning his filter. He immediately performed a massive water change, vacuumed the gravel thoroughly, and changed all the filter media.

Everything looked pristine, but the next day he discovered that half his fish were dead. Within the next week, the remaining fish died, even though he frantically performed several more water changes. He assumed that changing water killed his fish and is unsafe, even though numerous experts say differently.

What Went Wrong?

Did the water change kill the fish? The answer is yes, but not because water changes are inherently bad. The cause is more complex than that. Over time, the by-products of fish waste, uneaten food particles, dead leaves from plants, etc., alter the chemistry of the water. Because the fish live in the water and the changes happen gradually, they adjust to it.

When a sudden, large water change occurs, it causes such a drastic shift in the water parameters that the fish often cannot tolerate it and they die. Those that do not die immediately are stressed and may succumb to disease over the next few weeks. Cleaning the filter media also removes the beneficial bacteria that are necessary for breaking down toxic ammonia in the water. Naturally, the owner thinks that the water change and filter cleaning was the cause, and therefore, is a bad idea.

Should Water Be Changed?

If changing water can potentially kill fish, why do water changes at all? The answer is that regular water changes are important for the long-term health of your fish. The dissolved wastes in the water, which are not apparent to the naked eye, won't kill the fish outright, but as wastes gradually accumulate the stress reduces their immunity to disease.

Fish are exposed to bacteria, viruses, and parasites more often than owners realize. A fish with a strong immune system will rarely get sick even when exposed to pathogens. On the other hand, a fish that becomes ill usually has been stressed by poor water conditions and/or improper diet.

Even those fish that do not succumb to disease may be affected in other ways. Elevated ​nitrate levels are known to impact the growth of fish, as well as their ability to reproduce. Young fish are particularly sensitive to poor water quality. The best thing you can do to keep your fish healthy is to change part of the water regularly to maintain proper water chemistry. The regular partial water changes prevent the slow but drastic shift in pH, alkalinity, nitrate, and other parameters that affect the health of the fish.

How to Start Doing Water Changes

If you haven't changed your water for months, or possibly years, it is not wise to make a sudden massive change. However, you still should start changing the water regularly. Start small, changing 10 percent of the total water volume. Wait a week, then perform another similarly small water change. Continue this process for several months, each time increasing the percentage of water changed. This will subject your fish to a slow change in water chemistry, which they can adapt to without harm. Ideally, once you have performed those water changes, a 25% water change every 2-4 weeks will maintain good water quality in your aquarium.

You'll find that as you get used to the water changing routine, it takes less and less time to complete. It’s time well spent, as clean water is the key to keeping fish healthy and increasing their lifespan. Remember that the next time someone tells you that water changes are fish killers, when the lack of water changes is actually the problem.

Article Sources
The Spruce Pets uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Aquarium Fish FAQsFlorida Department of Agriculture and Consumer Services, 2020

  2. Beginner: Partial Water ChangesDuke University Extension