Why Fish Die After Changing Water, Find the Facts

Fish Die After Changing Water

The morning turns into mourning when you see one of your beloveds floating upside down. Oh, the poor fish is dead! Everything was right since you fed them one final time, said goodbye, and went to sleep. You wonder what went wrong!

Suddenly you realized you did a rigorous water change last day. So, did the water change kill your fish? Apparently, yes, but the actual answer is more explanatory than that.

Have you heard a tale of woe about the aquarium keeper who lost all his fish after a water change? He was doing fine without a partial water change even since he set up the tank over a year ago.

One of his friends noticed the substrate looks dirty, and the aquarium owner came to know he should have been doing regular water changes. The owner immediately took action, changed all the water, and cleaned the aquarium thoroughly.

Everything looked immaculate, and the owner was thrilled. But the next morning, half of the fish died, and the rest died within a week! The owner anguished to see all the dead fish and his advisor friend blamed tap water for being lethal. Was it really so? Let’s dig a little deeper.

Why did water change kill fish

The sudden alteration in water chemistry and aquarium temperature is always the culprit for fish’s immediate death after water changes.

An aquarium is a micro-ecosystem in which several biological and chemical processes are taking place continuously. Fish wastes, food leftovers, and dead plant matters alter the water parameters over time. If this change is slow, fish can somehow be accustomed to it.

An unanticipated drastic change in the water variables imposes tremendous stress on fish. It is more damaging if you haven’t changed the aquarium water for a very long time, and then replaced everything straight away.

Though you are adding the purest water to the aquarium, the pH, hardness, and other water parameters shift from the previous condition. Fish may not tolerate this change and die. Poor pets were attuned with some pollution, so the freshness is killing them!

Another potential reason for fish getting killed by water changes is a sudden down in water temperature. If the degree of temperature difference is vast, some fish may die going into shock, while others may succumb to disease over the next few days.

Inappropriate water changes are the slow killer of fish in another way. Over-frequent water change can kill off beneficial bacteria in the fish tank. Beneficial bacteria are essential to establish a nitrogen cycle that prevents toxicity from evolving in a fish tank.

If you remove all the water and thoroughly clean the aquarium, it could kill off the majority portion of beneficial bacteria. A fish tank that is deficient in beneficial bacteria requires recycling. Otherwise, toxicity builds up rapidly, and the water quality deteriorates. The entire process becomes harsh on fish and weakens them.

Pathogens presenting in the fish tank might not miss this opportunity to kill your fish. If your routine water changes are too frequent to allow growing beneficial bacteria, the result is the same.

Why should you change the water then

If changing water can potentially be fatal for fish, should you do it at all? The answer is, yes, you should do regular water changes for the long term health betterment of fish. Dissolved toxicity in the aquarium water not visible in naked eyes, cannot be eliminated unless adding some fresh water regularly.

Dissolved toxicities are slow poisons, not killing your fish then and there, but they reduce fish’s immunity and, eventually, the lifetime. Okay, let’s discuss a bit more in-depth.

Fish secrete waste, and it goes nowhere but accumulated at the bottom. While you are feeding fish, if they can’t take all the food within a few minutes, food leftovers join with fish wastes at the tank bottom.

In all feeding instructions, it is clearly briefed to feed fish as much as they can finish in five minutes, which is a very vital instruction as prepared fish foods like flakes or pellets don’t float for hours. If your fish don’t consume floating foods right away, it will sink at the bottom to be wasted.

All the aquarium wastes start breaking down at a point and decompose to ammonia. Ammonia is a highly toxic gas that persists dissolved in the water unless beneficial bacteria convert it to nitrates. The entire biochemical process is known as the nitrogen cycle, which is essential to be established in an aquarium.

The toxicity of ammonia is acute and fatal. Nitrates are less deadlier than ammonia, not killing the fish outright, still harmful, and has to be eliminated.

Live aquarium plants can consume nitrates naturally. Always, it may not be sufficient. The only effective way to remove nitrate from the tank is to do so literally through water changes.

Another crucial reason you should do regular water changes is that it can oxygenate the aquarium water. An oxygen-rich aquarium is a healthy environment for fish. Fish require oxygen just like you and us. They can’t breathe, but they have a distinct process of taking dissolved Oxygen (DO) of the tank water.

The aquarium water condition impacts a fish’s immunity system directly. A healthy fish with strong immunity will rarely get sick.

How to do water changes in the right way

Acclimatization is the key! Don’t inflict your fish on a sudden change. Despite a small risk that it might harm your fish, regular water changes are incredibly crucial. 

You cannot compromise subjecting your fish to poor water quality, and you cannot risk their life. So, you have to know how to perform water changes in a balanced way.

Small water changes regularly are pretty effective than a massive water change once in a while. We suggest changing 10-15% of the water every week and change the one-third monthly. It is advisable to use a gravel vacuum to suck out debris from the bottom once a month while performing the major water changes.

A gravel vacuum is a useful tool to perform water changes, so invest in a better quality tool. Siphoning does water changes along with cleaning a fish tank conveniently.

While you add new water to the aquarium, perform a simple water test to analyze pH with a testing kit. If you add tap water, don’t forget to treat the replacement water with conditioner and be sure it is free from chlorine and heavy metals.

Don’t forget to measure the water temperature with a thermometer. Always try to keep the new water temperature as close as possible to the aquarium water. For most tropical fish, the acceptable temperature ranges from 72° to 78° Fahrenheit.

Take away

Now you perceive that water change is not any dangerous business, but rather an absolutely essential part of fishkeeping. You have to perform it in a way so that fish can adapt to the change. A sudden change in water chemistry is the primary cause of fish deaths after water changes. But fish deaths after a water change is avoidable.

The key that gives the best result for a water change eliminating associated risks is a routine. When you follow a meticulous routine for water changes, your fish will be easily habituated with it. Good quality of water can keep your fish healthy and lively with a longer lifespan.

No more heartbreaking scenes of dead fish floating in the aquarium! Do regular water changes and do it correctly. Water quality in an aquarium is uncompromisable. Happy fishkeeping!

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Sujit Modak Aquarium Tales Owner
Sujit Modak

Hello, everyone! Welcome to my aquarium blog. Fishkeeping is my passion, and I started this fascinating hobby back in 2006. Besides my engineering profession, I deeply studied many fishkeeping topics since I started building my home aquarium. I researched effective aquarium filtration and lighting of planted aquariums. I am keeping 20+ species of freshwater and saltwater fish as my aquatic pet collection. I successfully experimented with a complex ecosystem inside the aquarium, biotope aquariums, aquaponics, etc. I would love to share some learnings from my hands-on experience of the last 14 years. Hopefully, my sharing will be somewhat helpful to make your aquarium journey awesome! 

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