Ooooo this makes me want to draw my betta fish who still doesn't have a name. He's opalescent... Shiny white all over. I've never seen a betta fish solid white and opalescent, but he's beautiful, and very spunky.
Anyways, enough about things that aren't your art lol. This is a beautiful piece. I love the softness of it all and the purple shading in the blue parts of his fins. It really brings a multidimensional feel to the piece. Overall, it is a beautiful painting and one that makes me interested in seeing the rest of your gallery. The commissioner is a lucky one, they got a very good piece that is worth whatever they spent.