Are you wondering, why did my fish die after a water change? Fish are sensitive creatures that don't like rapid changes. Their watery home needs to stay consistent for them to be happy and healthy. So, if you don't perform water changes correctly, it can make your fish go into shock, become sick, or die. If you don't do it right, you might experience fish dying after water change. Large water changes can kill fish if you kill all the bacteria. It can also happen if you don't match the water temperature, pH, and water hardness. All Fish Died In A New Tank If all your fish died in a brand new tank, it may be because you didn't cycle the tank first. Before adding fish, all tanks need to go through the 'nitrogen cycle'. The nitrogen cycle usually takes between two weeks to two months to complete. You should avoid adding fish to the tank during this time. There is such a thing as a 'fish-in' cycle, but I don't recommend this as it's stressful for the fish. Instead, to jumpstart the cycle, add a bit of fish food to the tank. The fish food will begin to break down and convert to ammonia. Ammonia is toxic to fish. As time goes by, the ammonia will break down into nitrites, another toxic substance. Lastly, the nitrites will break down into nitrates. Nitrates are toxic at high levels but are not as dangerous as nitrites and ammonia. What causes these substances to break down? Beneficial bacteria. Beneficial bacteria are the backbone of every healthy and successful aquarium. Without it, your fish aren't likely to survive for long. Without bacteria, these things can't break down. The ammonia, nitrites, and nitrates continue to build up. The tank becomes a very toxic environment for your fish. When you first set up a tank, there are no beneficial bacteria. So, your fish create waste, but there is nothing to break that waste down. So, your fish gets stressed and sick. Often, the fish will die. Allowing the bacteria time to colonize is why you need to wait about a month before adding fish to a new tank. The RSPCA says that "Beneficial bacteria grow on any surface in the aquarium and they are concentrated in the filter sponge/media which has a high surface area."