As a poaching liquid I have used a fish stock made from the bones of the fish that I am poaching. I normally buy the whole fish because I can really see the freshest and do other things with rest fot the fish. That is if I am poaching the fillet.
If you are in California Aisan market will dress the fish for you and I the staff deep fish head and bones to get more favor. Then after getting home I make a stock with the bones with carrots, celery and onions chopped.
I normally will have enough broth to make a tofu soup with some of the trimming of the fish. Adding a little baby shrimps, scallops, vegetables, tofu and whatever I have around the house.
A complete meal with just a whole fish.
Lucky you. If it's the All-Clad like my sis', then it's a beaut. I've never poached a whole fish since I don't have a long poacher, only done salmon fillets in a saucepan. Don't know if you can poach whole fish; makes sense that you'd have to fillet fish in half lengthwise. I don't have any specific poaching liquid recipe to offer you, but general tips:
Include balance of flavors: my liquid contains mostly H20, but I usually add: apple cider vinegar, brown sugar, whole peppercorns, maybe juniper berries, salt, maybe some white wine/sherry and fresh herbs (thyme or rosemary sprigs).
I read somewhere that you want to put fish in w/ liquid as it's heating up, instead of adding when it comes to low boil. Believe this provides more gentle cooking. When water comes to low boil, I turn it down to very low simmer and cover w/ lid. I forget poach time, but I believe it's around 7-8 min./in.
re: Carb Lover
You can also turn off the heat once it comes to a low boil.
It helps to place greased/sprayed wax or parchment paper (or foil) directly atop the fish and liquid. This reduces the likelihood of too much steam cooking the fish (steam is too hot for poaching and toughens the flesh). Simply trace the bottom of the pan on the paper or foil and cut out a form.
Don't cover the fish in liquid. The liquid normally only comes about about a third of the way in height.