Goldfish may die after a water change for the following reasons:
- The goldfish sustained injuries when being moved before or after the water change
- Too much water was displaced and changed
- The water’s temperature dropped significantly
- You used untreated tap water
- You cleaned the substrate and filter simultaneously, ruining the nitrogen cycle in the tank
- You suddenly changed the water after not having done so regularly
Despite the common misconception, goldfish don’t die easily when taken care of properly. Water changes are necessary to keep your goldfish healthy, but executing them properly can be tricky.
If your goldfish dies after a water change, you should revisit how you did it and see if you accidentally made any mistakes that might have harmed the fish.