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I just got a turtle and he is about 4 1/2" long. I feed him around 15 pellets a day. Is that good?

That's hard to say. It depends on the size of the pellets. Sounds like a lot though, probably too much.

Here's how to determine the correct amount: Imagine your turtle's head is hollow (not including the neck). How many pellets would it take to fill the hollow head? That's how much your turtle should get every other day. Or, if you feed him every day, feed him half as much.

Here's the excerpt on pellets. While you're at it, why don't you read the whole nuitrition section. It's full of good tips:

http://www.redearslider.com/pellets.html

Unless the pellets are micro pellets I'd say that's a bit too much. What brand pellets are they? What's the protein content?