Most likely they have developed tropical fish disease. There are hundreds of tropical fish diseases but having some knowledge of a few of the most common ones can help save your fish.
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Types
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There are two main causes of tropical fish disease. They are ineffective management of the fish tank and introduction of disease to the tank by new fish. Most aquarium owners put a lot of time into making sure the tank is the proper temperature and excellent for the fish. So introduction of new fish also called "new tank syndrome" is the number one cause of the two for tropical fish disease. Other diseases include fungal infections, eye fluke, whitespot, gill worm and fin rot.
Identification
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Each disease has its own identifying symptoms and signs. When a fish has new tank syndrome he stops eating or sometimes will die. This disease is usually caused by stress during transportation to the new tank. Eye fluke is caused by chlorine poisoning or dirty water. The fish's eyes will have a white film. Gill worm is easy to identify as there will be visible worms hanging from the fish. It is a parasitic disease. Frayed fins are a signal that fish have fin rot disease. If a fish is not treated the fin rot will go into a secondary more serious infection. Whitespot will show itself in small white spots on the fins and head of the fish. Other typical signs of various tropical fish diseases are loss of appetite, cottony like covering over the body and fish that have red streaks, seem to be gasping and seem to be sinking when they try to swim.
Treatment
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Treatment varies depending on the illness. When a fish is sick a first step is to test the water. You can get water testers in any pet store or aquarium store. One thing to try on many tropical fish diseases is a salt bath. One teaspoon of salt is added to a gallon of water and the fish swims in it for the healing properties. Other illnesses require prescription medications from a certified veterinarian. There are also anti parasitic medicines available at pet shops. It is best to at least talk to a veterinarian about your fish's symptoms because some may require medication in addition to the non medical remedies to avoid secondary illness or death.
Prevention/Solution
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The best way to prevent fish disease is managing the tank well. Clean the tank on a regular basis. Snails can be used to keep the tank clearer between actual cleanings. They will eat built up bacteria but the tank will still also need to be cleaned properly. Use a filter on the tank also. When introducing fish to the tank some basic but important steps should be taken to do it successfully. Ignoring these steps can upset the ecosystem in your tank and cause illness. Isolate a new fish for a couple of weeks before introducing it to the tank. It should have plants and things to hide behind because fish do not like being alone and it can be stressful. The plants provide stress relief because the fish likes to hide in things. Treat the new fish with antibiotics and anti parasites from the pet store. Even a healthy fish may have unnoticed illness and this will insure your fish is as healthy as possible before adding it to the tank. Educate yourself on tropical fish diseases. There are books available on tropical fish keeping and more specifically tropical fish disease. Keeping one available for reference is a good idea.
Warning
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When buying fish food always buy one that has tropical fish on the label. While they will eat regular goldfish food, food especially for tropical fish will keep them healthier. Follow feeding guidelines and do not overfeed. When using medicine additives to your tank, such as anti parasitic ones, keep in mind that it also destroys healthy bacteria that fish need. Use medicines sparingly if they are bought from a store. If the veterinarian suggests or supplies medicines, follow directions exactly. Killing too much good bacteria can make your fish just as ill as the bad bacteria causing the illness so use caution in the amount and length of use. Salt water baths will often achieve the same results without killing off the good bacteria in the tank.
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