Definitely a fun and relaxing game. I like the sound effects and the different colors. However, I think the background is a bit too noisy. Also there is not much contrast between the red fish and the background of the hell difficulty. This makes it sometimes difficult to see the red fish.
I think also some more variety would work well. For instance, you could add wooden boards or something that swims around and blocks the antenna when hit. That way, you carefully need to navigate around those obstacles.