Probably 2-3 litres of water. I think one litre equals one quart, if I'm not mistaken. So about 1 drop per liter. It's better to put less, see if they like it, and then increase.
He eats once a week. You're right, I forgot to mention the shell - you can see the beautiful yellow marks on his shell - they are new, showed up after the iodine. He LOVES it.
Thank you for your response a couple of days ago. We are still mourning our friend and co-worker. I/we can't believe she's gone. So young and so wonderful!