How deep do you have the water? Maybe he/she's just pooped out. If the water's too deep, he'll need something to cling to. Or make a shallow area.

Hatchlings should only be in about 2-3" or water, thought I've gone a little deeper.

Besides from this, if he/she can swim it's fine. Staying at the bottom is a symptom I've never encounterd though, so check with others here.