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Feed your goldfish 2 or 3 times a day. Feed them only a few pinches of food each session. Be sure you’re feeding your goldfish nutritional feed that is designed for the particular type of goldfish you have.  In total, provide an amount of food over the course of the day that allows for 5 to 6 minutes of feeding. If the food is dry, soak it in water from the tank for five to ten seconds before feeding your fish. Un-soaked food can injure or sicken the fish when it expands in the stomach. Favor pellets, which should sink, but also include flaked fish food – which will often float on the surface, as goldfish like to eat from both the surface and the bottom of the tank. While feeding goldfish, follow the classic "less is more" approach, as overfeeding a goldfish leads to far more dangerous complications than underfeeding.  Watch for long trails of poop trailing from behind your goldfish.  If present, you're feeding it too much. Your goldfish should usually be fully aware of its surroundings and be moving around with interest and excitement.  Most of a goldfish’s time is spent nibbling on stuff and actively swimming around.  If your goldfish isn’t doing so, look closely for signs that it may be sick.  Goldfish eyes should be entirely clear.  If your goldfish has foggy eyes, it is likely sick. Check out the fish’s fins and body.  Fins should be stiff and solid, without uneven edges. Look at the goldfish’s skin.  White spots, fuzzy patches, or red streaks all indicate illness. Test your water at the first signs of illness.  Often, an illness is a sign that either water or feeding needs to be adjusted. Whether your goldfish has developed unsightly spots, is floating around erratically, or has fins that are not as sprightly as usual, there are proven methods of treatment for the most common goldfish illnesses.  There a handful of common goldfish diseases, most of which can be treated by refreshing the water, though some require more detailed treatment. Identify the particular sickness your goldfish is suffering from by crosschecking its symptoms with online descriptions and treat your fish accordingly. Check out the various wikiHow articles on how to treat common goldfish health issues such as swim bladder complications and white spots.

Summary:
Feed your goldfish a well-monitored, nutritious diet. Take care not to overfeed your goldfish. Watch out for signs of illness or infection. Diagnose your goldfish’s illness by consulting specific symptom lists online.