Diseases
You can easily tell when a fish is falling off in his health by observing him frequently coming up to the surface of the water for air. This shows he has not sufficient power in his gills to extract the air from the water. He also looks dull, and his motions are languid; a hazy or cobwebby appearance likewise seems to envelop his body, and perhaps some of the scales will drop off. When a fish gets into this unhealthy state, he should be immediately removed from the others, who should have fresh water given them several days in succession. The best remedy for diseased fish is to put them into a pond for a few weeks; and it is especially necessary for female fish, which, if not so treated, frequently die for want of spawning. A fish is sometimes saved by being placed in a little artificial dam, made from some running stream in a garden, for two or three days; but their diseases are at all times very difficult to remedy. The best way is to prevent them by the precautionary measures--plenty of room and pure water.