It sounds like your using an airflo poly-leader or rio sinking leader.
Use only tippet material with these leaders and not a full-length leader like recommended above. A long leader like you'd normally use will not allow the fly to get down with the sinking leader nearly as well as a shorter one. If you use a normal leader it will reduce the depth that your fly fishes on the sinking leader -that's the point of having the sinking leader -right?
Don't use too long of a tippet either; you want the fly to fish deeper in the water column. You'll probably want to use about 1-1.5m of tippet. Using a shorter tippet is even more important in turbulent streams like the step-pool waters that you plan to fish. A loop tied at the end of your poly-leader is a convenient way to attach your tippet.