I add salt, and quite a bit, to the water thawing the fish.
It seems to cut the slime quite a bit and I feel it helps keep the bacteria down to a low roar since some will be there awhile while cleaning others.
That many I'd probably change out the water after a quarter or a third of them and start over with another batch and fresh salt water.
Just my 2 cents.
God Bless Heading out tomorrow hoping for a long afternoon bit.....lol