I'd suggest you don't use it as a topping and instead use the tomato juice to braise the fish.
If the fish is a fillet, cut it into two inch pieces, if it's not a fillet use your best judgement...
Stand two of these up on end on a pice of foil, top with some sliced tomato, extra virgin olive oil, kosher salt and fresh ground pepper. Top with another pice of foil and seal the edges. Bake on a cookie sheet at 350 or so for like 15 min. You'll have to try it first to see how long things really take.
But as long as you have good fish, a decent tomato (use a firm Roma if off season) and nice spice it will taste excellent. This is a very easy and good way to cook fish, especially salmon.
As always, it's about what you're putting into the dish. Good stuff in = good stuff out.
thanks for asking
-spence