肉 and 魚: meat, fish as food, and living fish
Read ingredient names and meanings outside food according to context.
Meat as an ingredient
肉 (にく) means meat in a food context. 「肉を薄く切りました」 says that the speaker sliced the meat thinly.
「肉は食べますが、魚は食べません」 says the speaker eats meat but does not eat fish. Keep the positive first clause separate from the negative second clause.
Living fish and fish as food
魚 (さかな) can refer to living fish or fish as food. 「川で魚が泳いでいます」 describes fish swimming in a river.
「夕食に魚を焼きました」 says that the speaker grilled fish for dinner. Context distinguishes a living animal from food being prepared.
Narrowing the meaning through context
「腕に肉が付きました」 means gaining flesh on the arms. Do not always interpret 肉 as an ingredient.
「魚を一匹買いました」 says that someone bought one fish. The counter 一匹 alone does not determine whether it is a pet or food.
What does 魚 refer to in 「川で魚が泳いでいます」?
Show the answer and why
It refers to living fish swimming in the river, not a cooked fish dish.