Topsoil would be the perfect preventative to turbid water. Can you spread it with a dozer for good compaction and seedbed? While you're at it, spread some gravel and chat a few inches thick in areas that will be 2-6 feet under for spawning beds. Ag lime can be added at any time but is easiest before filling. If it is already full, try a speader and go around the edges extra heavy. Ag lime cannot raise the Ph too high so you can never apply too much. Still you don't want the PH to rise rapidly and this should not be a concern with ag lime.
Floating clay IS hard to grasp. Suspended clay (colloidal clay) are extremely fine particles that are NEGATIVELY charged electricly and act like magnets of the same pole to repel each other. They are so small they are nearly weightless in the water and the repeling effect keeps them floating. Alum provides a plus3 POSITIVE charged atom that attracts several of the clay particles to it and allows it to "clump" or "Floc". These "Clumps" then quickly gain weight and settle to the bottom. Most decaying organic matter (Detritus) also provides positivecharged Ions to help neutrilize the negative charged colloidal clay.
Class is dismissed! If you knew how badly and slowly I type, you would never ask me questions like this!