Gas Bubble Disease
From The Aquarium Wiki
Cold tap water is under pressure and contains chlorine gas so when it comes out of the tap the drop in pressure will release these gases trapped in the water.
- Rapid heating of cold water causes the gas to escape more rapidly and it adheres to all surfaces. This includes the inside and outside of your aquatic animals! If you could examine the gills of the fish you would see tiny bubbles of gas sticking to them which causes lethargy, brain damage and death.
This is because the bubbles on the gills prevent fresh water with its oxygen to penetrate the gills and so the animal in effect suffocates. Also the gas penetrates the gills and the gut and so enters the bloodstream to form bubbles and so blocks the flow of blood. Similar effect to the 'bends' divers can suffer.
Tiny bubbles on surfaces and on the bodies (skin, eyes, fins) of the fish. They may appear slow and lethargic.
- Aerate the water to force the dissipation of the gas.
- Do not use fresh cold tap water. Heat it up first and let it lie exposed to the open air for several hours.
Place a dry finger or similar object into the water for a minute and observe if bubbles form on it.