That's because the one-time water change is too much, the water quality and water temperature can't let the fish adapt, and the phenomenon of fish going upstream. The larger fish or stronger resistance, has improved the ability to adapt, so it is quieter and sinks below.
When changing water, pay attention to the amount of water not more than one-third, one-quarter of the best, as little as possible to change the fish's original water environment. Because ornamental fish are very sensitive to water quality, one-time water change too much on the original water quality impact. When using tap water to change water, you can dry the water two days in advance to let the residual chlorine in the tap water evaporate.
Expanded Information:
After buying the fish back, don't rush to put the fish into the tank, you can put the fish bag into the water together, first let the fish to adapt to the temperature of the tank, then add the water of the tank to the bag with fish, and then continue to let the fish to adapt to a period of time, and finally put into the tank.
Poor water quality fish are easy to get sick, such as fish white spot problem is also mostly due to water quality. Water quality stabilizer can be used. The problem of rotting is usually caused by bacteria, you can put in gentamicin, potassium permanganate and so on to disinfect the fish tank.
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