I used to do the same thing- stick them under the floor in the blood bin but found it still makes a big difference that 5 minutes. The quicker the fish is killed and put in a slurry the better. You end up with a bit of blood in the ice anyway so doesnt really make much difference letting them bleed out.
Also the ice slurry makes a massive difference to just ice.
You can see it in photos- the fish that have just gone on ice will have white marks where they have been touching other fish and the blood has pooled. You get them into the ice slurry and the colours are vibrant and the flesh is much better for it.
Here is a photo of the last time i went to the reef. These fish had been in there a full day and were just taken out for a quick photo and transfer into an esky out of the boat for filleting (cant lift 200kg of fih and ice out of the boat!) Check out the colour of them.
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