如何在JFrame上显示带有图像的两个JPanel，并且两个img都可见？

Question

我试图将水族馆（这是一个延伸JPanel和包含水族馆img的类）设置为背景，并在顶部放一条鱼（这也是一个延伸JPanel并包含鱼img的类）。

问题是它在水族箱顶部只显示一个图像而不是鱼（水族箱或鱼，取决于哪个首先添加到JFrame）。

主要

public class Core {
    JFrame window;
    JLabel label;
    ImageIcon img;      
    Aquarium aquarium = new Aquarium();
    JavaFish javaFish = new JavaFish();

    public void start() {
        window = new JFrame();
        window.setPreferredSize(new Dimension(600, 400));
        window.setVisible(true);
        window.setTitle("Java Game");
        aquarium.add(javaFish);
        window.add(aquarium);
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

    public static void main(String[] args) {
        Core c = new Core();
        c.start();
    }
}

水族馆

public class Aquarium extends JPanel {

    private BufferedImage img;
    //Initiate aquarium width 
    public int width;
    //Initiate aquarium height
    public int height;

    @Override
    protected void paintComponent(Graphics g) {
        width = getSize().width;
        height = getSize().height;

        try {
            img = ImageIO.read(new File("img/AquariumBackground.png"));
        } catch (IOException e) {
            e.printStackTrace();
            System.out.println("Image not fount!");
        }

        g.drawImage(img, 0, 0, width, height, this);
    }               
}

鱼

public class JavaFish extends JPanel {
    BufferedImage img;
    int xPos = 50;
    int yPos = 50;

    public JavaFish() {
        this.setOpaque(false);
    }

    @Override
    protected void paintComponent(Graphics g) {
        BufferedImage JavaFish = LoadImage("img/JavaFish.png");
        Graphics2D g2d = (Graphics2D) g;
        g2d.drawImage(JavaFish, xPos, yPos, 100, 100, null);
        repaint();
    }

    BufferedImage LoadImage(String FileName) {
        img = null;
        try {
            img = ImageIO.read(new File (FileName));
        } catch (IOException e) {
            e.printStackTrace();
        }

        return img;
    }
}