Matisse was known for his use of bright colors and bold, flowing lines, which he felt conveyed a sense of joy and vitality. He often painted still lifes featuring goldfish swimming in bowls or ponds, which he saw as symbols of peace and serenity.
To Matisse, goldfish represented a sense of calm and harmony, a respite from the chaos and stress of everyday life. He believed that by painting these peaceful scenes, he could create a sense of tranquility and balance in the viewer.
So, my dear friend, the answer is "a tranquil state of mind".