33、Many people keep small fish as pets. They keep them in a tank of water. The tank is made of clear glass. People can look through it. They can see inside the tank and watch the fish.
The fish need room. They must not be crowded. Fish need oxygen (氧气) to breathe. Fish get it from water in the tank. There must be enough water for all fish. So the size of the tank is important.
In the tank many people put small plants that are good for fish. They give oxygen to the water. Plants help in other ways，too. Fish can nibble (轻咬) on the plants. They hide among the plants and sleep，lay eggs there.
The plants need light. Most fish need light too. The tank may be put near a window. It will get light there. Another place might be better，though. A window may let cold air in. The water in the tank may get too cold. The fish may die because cold water can kill them. So can hot water. The temperature must be just right. The fish need food and they should be fed every day. Giving them too much food is bad. The extra (额外的) food will fall to the bottom and make the water dirty. However，the fish should get just enough food. They should finish the food in 10 minutes. Nothing should be left. When this happens，the amount (数量) of food is right.