I'll purge with salt, even if Dr. Coonass says it doesn't do anything. Every bayou living coonass I've ever seen cooking crawfish uses it, and even if it's a wive's tale, a can of salt is only a dollar or so. Why not?
I do the same setup, but I have two purge stages. Both with salt. So, first into tub 1, then to clean water and tub 2, then into the pot. Tub 2 water is WAY cleaner than tub 1 water, for obvious reasons. Makes me feel like I got them cleaner.