(1) Fish activity: Healthy fish usually swim in groups, with flexible actions and quick response, and dive into the water immediately after being frightened. However, sick fish usually swim alone, which is slow and unresponsive. After being frightened, they will swim deeper into the water, or swim to the surface again soon. When some parasites are parasitic, fish will spin wildly on the water, or keep jumping on the water.
(2) Fish feeding: Healthy fish have a big appetite and strong ability to grab food. When feeding, they gather in groups around the feeding ground to grab food, and their food intake is relatively stable, while the appetite of sick fish decreases, and their food intake will suddenly decrease, or they lose their appetite and don't eat at all.
(3) Body color change: All kinds of fish have their own unique body color, which is bright in color and strong in refraction, but when diseases occur, the body color of fish is usually black or grayish white; Some sick fish have red bleeding spots, bleeding spots or white spots on their bodies.
(4) Body shape and body surface changes: healthy fish have thick muscles, good fatness, complete body surface and a layer of mucus, while sick fish may appear shriveled, ulcerated and bent, or have foreign bodies attached, the mucus on the body surface increases or decreases, scales fall off or stand upright, abdominal swelling, exophthalmos, anal swelling and so on.
(5) Anatomical changes of fish: Most diseases show different pathological changes of various tissues and organs, such as bleeding, discoloration, swelling or atrophy. For example, the gills are bright red in normal times, but when bacterial gill rot occurs, the gills are rotten with sludge or mucus, and the gills are purple or pale; In bacterial enteritis, the intestinal wall becomes thinner, the elasticity and toughness decrease, the intestinal cavity is filled with a lot of mucus or bloody mucus, and the mucosa is necrotic and bleeding.