My first thought is that this is a pretty frickin huge canvas for a pixel art piece. And if you're going for a detailed, photorealistic style, then you've got a whole lot of work ahead of you. I imagine the background could turn out to be more work than the fish, really. Personally, I would make this smaller, especially since you're not quite sure about the technique you're going to use for the background.
I've started a fair number of pixel art pieces that were too big, without really knowing how I was going to pixel them. Then you spend a whole lot of time making changes every time someone offers advice, since the canvas is so huge, and eventually you run out of steam.
Just my humble opinion. If you're really eager to work on a big piece like this and put in the hours, I wish you lots of luck. Unfortunately, I can't really offer any constructive criticism yet.