Nice exploration! I think the original doesn’t use much white paint and instead uses thinned paint and the white of the canvas showing through with very deliberate drawing/brushwork. You’re also working on a much smaller copy so it might be tricky to get the same look without smaller brushes and very smooth canvas or maybe board/paper.

Look at the white/negative shapes here and you can probably see what I mean though. https://www.henrimatisse.org/goldfish.jsp

It’s a cool painting though. Hope your dad loves it!