Flaky fish food floats at the top of the water, so when fish take a bite they also gulp down some air. This can cause their organs to become enlarged, leading to swim bladder disorder. Try soaking fish food before adding it to the tank so that it will sink into the water, allowing fish to eat it without taking in air.  You can also buy sinking fish food that automatically sinks to the bottom of the tank without needing to be soaked. If you feed the fish anything besides flakes or pellets, make sure it's nutrient dense and fully thawed before feeding. When fish eat too much they can become constipated, leading to intestine or stomach enlargement and swim bladder problems. Fish should only be fed a small amount of food once per day. Even if your fish seems like it's always hungry, it only needs a small amount of food to function healthily. A dirty tank harbors bacteria and parasites, putting strain on a fish's symptom and sometimes leading to infection. Make sure to clean the tank frequently so that your fish is taking in clean water, rather than swimming around in waste.  Use a water test kit to check pH, ammonia, and nitrite levels. Changing the water doesn't guarantee proper levels, especially if you have never tested your water since starting your tank. Goldfish do best with a pH level of 7.2 - 7.6, as little ammonia as possible and a nitrate level of between 0 and .25 ppm. Try adding aquarium salt made for freshwater tanks. Aquarium salt is good for helping to fight disease and boosts the goldfish's immune system. Check the temperature every so often to make sure it is about 70 °F (21 °C). Goldfish don't function well in cold water; keeping them at a lower temperature can strain their systems and slow digestion.

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Write a summary of the previous text in Vietnamese:
Ngâm thức ăn trước khi cho cá ăn. Không cho cá ăn quá nhiều. Giữ cho bể cá luôn sạch. Duy trì nhiệt độ nước thích hợp.