Why Fish Die: There are multiple reasons behind fish dying in the tank after water changes, but the most important of all them is “shock or stress”. If yours is a tropical fish tank then you have to be very careful “before” you plan a big water change. If you do not have the facility of storing water, before you add it to the main tank then you can even start treating it with commercially available products that treat water to desired chemical properties.

What are the reasons fish die?

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.

Water parameters causing fish die

1. pH
2. Temperature
3. Alkalinity/Hardness
4. Ammonia
5. Nitrite
6. Nitrate
7. Chlorine/Chloramine

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.2 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.

PH changes

Any time you attempt to change pH levels in your aquarium, remember that fish are very sensitive to pH changes and if it's done too rapidly, it can cause extreme stress or even death.

You need to be more careful about the pH level in the water. It's better to check the pH level in the water before and after the water changes.

Temperature

Fish are poikilothermic; therefore temperature dramatically affects their metabolism, including immunity. A decrease in water temperature suppresses the immune response. Perturbations in immune function may partly explain why many pond fish diseases are most common in the spring and fall when temperature fluctuation is greatest.

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Alkalinity/Hardness

Definition of alkalinity: "The buffering capacity of a water body; a measure of the ability of the water body to neutralize acids and bases and thus maintain a fairly stable pH level

Tap water has low alkalinity and hardness if we do large water changes. Your tank water becomes very soft and if has no ability to buffer pH in the tank system and it leads to water being more acidic

Ammonia

If a filter is a correct type and size for an aquarium, you shouldn't need to clean it more than once every 3-4 months. Cleaning aquarium filters correctly ensure the established beneficial bacteria are not disrupted too much causing an ammonia spike in your aquarium.

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Nitrite

If a filter is a correct type and size for an aquarium, you shouldn't need to clean it more than once every 3-4 months. Cleaning aquarium filters correctly ensures the established beneficial bacteria are not disrupted too much causing Nitrite in your aquarium.

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Nitrate

Large water changes reduce nitrate in the tank. Nitrogen in the shape of nitrate is an important nutrient for aquatic plants. If the growth conditions are good, nitrate - among other elements - is consumed by the aquarium plants.

Chlorine/Chloramine

Although chloramines help to make your water safe to drink, they are toxic to fish and Invertebrates. Chloramines must be removed from the water these animals live in. Unlike free chlorine, chloramines do not dissipate rapidly from water, so you will need to take extra steps to remove them.

The simplest and most effective way to remove chlorine and chloramines from your water is using chemical de-chlorinators. There are many products sold for aquarium use that is specifically intended to remove chlorine.

How many days should I change the water in my fish tank?

Change 10 to 15 percent of the water each week. If your tank is heavily stocked, bump that up to 20 percent each week. A lightly stocked tank can get by for two weeks, but that should be the maximum length of time between water changes as you do not want to place any stress on your fish.