Pete,
Good signs so far.
There doesn't appear to be much die off.
Now might be the time to manually move the rock, and siphon/drain the detritus.
Make up a large container (same size as the drums you have the rock in now) of fresh salt water, and have another bucket of fresh saltwater on hand.
Remove each piece from its current drum. Plunge it up and down a couple times in the bucket, and give it a shake under water. You'll be surprised at the junk that comes out.
Place all rinsed rock into the new container of fresh saltwater. Empty and refill the rinsing bucket as required.
Then drain and clean the original tub the rock was in, and refill it with fresh salt water, ready to receive the rinsed rock from the next original drum.