I have one that sits at the bottom of my tank, and my turtles ignore it excluding a bite or two off of it every three to four days. I like to have the whole in there as it helps with their beaks (gives em something to chew and scratch their claws on

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If all they are going after is that take it out for a while, and introduce it again later. You definitely do not want them to over eat it. To much of a good thing is a bad thing and such.