We've fed our turtles both ways. They were a little more shy about feeding out of the tank when younger but now they'll eat anywhere...they're pigs lol to make it easier for your little ones to find since they're soooo tiny in such a huge tank the donut may work. My suggestion though is to get a good digital scale and keep an eye on their weights so you can see if they're eating. If they drop weight you may have to start them in a smaller tank or put them in smaller bins to feed.
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