Any time your fish exhibits unusual behavior, such as refusing to eat, it's an indication that something out of the ordinary is taking place. Make sure you’re feeding your fish the appropriate type and amount of food for their breed and size. If problems persist, consult an aquatic veterinary specialist for additional information.
Live-bearing fish may not eat as much as usual right before giving birth. If you suspect your fish is pregnant, move her to a breeding tank where she can have privacy and protection and a secure environment in which to give birth. Typical signs of live-bearing fish pregnancy include a swollen abdomen and a dark triangular marking toward the lower fin. You may even be able to see the dark spots of the fry eyes through the mother’s body.
A fish that doesn't eat may be sick. Watch your fish for other signs of illness, such as staying in one part of the tank, appearing listless or swimming abnormally. Old or sick fish may begin to list or swim on one side. This could be a natural age-related progression of an older fish or a sign of illness that can be treated. Consult a vet for an evaluation.
Stress can sometimes cause a fish to not eat. Stress can occur when a fish is transferred from one tank to another for cleaning or moving, or if the tank temperature suddenly changes. Saltwater fish are more susceptible to temperature-related stress than freshwater fish. The addition of other new tank mates can also temporarily stress your fish and shy them away from food. Limit the stress on your fish by conducting regular partial water changes rather than conducting full-scale cleaning. It will reduce stress and help you maintain an overall healthier tank environment.
Fear can prevent a fish from eating. This is often observed in a community tank in which larger, more aggressive fish frighten away smaller, docile tank mates when competing for food. To help alleviate this problem, choose fish for your community tank that are relatively the same size and temperament, or feed adequately so competition for limited food resources is reduced.
If you’re feeding your fish too much, it may appear that they aren’t eating. Make sure you’re providing the appropriate amount. Watch your fish when you feed them to assess consumption and behavior. If a vet rules out health issues, try different brands of food on picky eaters to see if appetite improves.
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