If you notice that your goldfish is not behaving normally, but instead is hanging around the top of the tank or near the bottom and is not swimming around much, he is probably lethargic. This is a behavioral symptom that is a sure sign that something is wrong. Once you observe that your fish has become lethargic and after you've checked your water parameters, you have to look for other symptoms since lethargy is not specific enough to provide a definitive diagnosis.
A lethargic goldfish is spending an unusual amount of time in the bottom corner of the tank. You can also tell he is not feeling well if his dorsal fin is not erect.
CAUSES and REMEDIES
If your fish is lethargic, this may be due to poor water quality. Check your water parameters and perform water changes if necessary. These tests are best done with your own liquid test kits. As a minimum you need to test for ammonia, nitrite, nitrate, and pH. If you need help interpreting the tests, please visit GAB's Water Quality Forum.
If your fish is not moving around much, it may be because the water is either too warm or too cold. Take care to measure the temperature. Tropical fish like a temperature around 78-79 F. For many tropical fish anything lower than 77 F may slow them and cause them to become stressed. Goldfish will do well in temperatures from 68 to 79 F. Most aquarium fish can tolerate temperatures up to 82-84 F, if you provide plenty of airation (warmer water holds less oxygen). To minimize stress, change the temperature of the water in your tank slowly/gradually.
If your fish is lethargic, check for other signs like redness, fuzz or fin deterioration that would suggest bacterial causes. If you cannot find any such signs (and you have ruled out water quality, parasites, egglaying, or dropping fry), check to see if your fish is still eating. If he is lethargic and not eating, it is probably an internal infection and it would be prudent to move him to a hospital tank. Feed antibiotic food (like Medi-Gold) or if your goldfish is not eating, treat with a broad spectrum in-tank antibiotic like kanamycin. If your fish is big enough you should consider injections. Keep checking for other signs of illness.
Parasites can cause lethargy - particularly parasites that attack gills such as flukes or costia. Check carefully for signs of these parasites by looking at the slimecoat and seeing if your goldfish is gasping or flashing. Also if possible check the gills.
Lethargy may also be a sign of internal parasites. These are hard to diagnose as they do not leave many clear symptoms except lethargy, failure to thrive and losing weight despite eating. If you have access to a microscope, you can try to identify potential pathogens by looking at some poop emulsified in tank water. Treat for internal parasites with Jungle Anti-Parasite Medicated Fish Food. Read more about treating for internal parasites here.
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