The sauce chef is generally regarded as the most important member of any kitchen brigade. He is the person who can make or break a good meal more than anyone else. To make a good sauce takes practice and, moreover, patience. A really first-class sauce takes time and, most importantly, it also requires perfect ingredients and excellent base stocks.
Concentrate on making a good stock, in this case fish stock. All it needs is 20 minutes cooking time, but careful preparation of the bones and the vegetables, and nursing while it cooks will result in a finished product to be proud of.