I like the concept and the game itself looks vibrant and quirky with some nice music. Of course, it would be nice to have more levels and additional mechanics like an air meter where, once it runs out, costs you a life. One thing to consider also is to have a button where you switch between fish instead of controlling them both at once.
Overall, though, this has a ton of potential and I would love to see it become a full game.
Coincidentally, I made a goldfish game, too. Want to try it out?