The goal is to have bacteria dissolve the fish faster than it can spoil. You shouldn't have any fishy aromas. As explained above I let a compost reach at least 110 degrees before adding fish. When the pile reaches temperature I turn it while adding in a layer of fish between layers of other materials. Whichever bacteria are involved with fish reproduce extremely rapidly.
Bacteria appear to produce enzymes that dissolve fish.
In three to five days there isn't a trace. Sea materials are high moisture so the mix of browns should be different shapes and textures to allow for airflow and somewhat absorbant. Leaves by themselves will cake up after fish dissolves. Fish requires more experience and finesse but is easy and quick to work with once you get a system down. Hot composting or burial are the only ways to compost fish.