What happens to goldfish if the water is changed at low temperature?
Expert Answer
If the water is too cold when changing it, goldfish may show stress—swimming restlessly and fast, jumping out, or bumping the tank. Also, a sudden temperature drop weakens their immune system, making them prone to diseases like white spot and fungus. Finally, a sudden drop affects digestion, causing lowered appetite and indigestion, even enteritis or dropsy. So when changing goldfish water, keep the new and old water within 2°C of each other, and add the new water slowly along the tank wall to avoid shocking them.
I learned to match new water temperature when I shocked my goldfish once, and the stress plus immune dip gave him white spot, so I match temp and do partial changes. I'd never dump cold water in, the swing is what hurts. Mine's healthy since I stabilized the routine, but I'd treat temperature matching as basic, a sudden drop invites disease, gentle consistent changes keep fish right.