Red Ear Slider very sluggish

Question
Last week I bought 2 baby turtles, one red ear slider and one ridgeback. The ridgeback is extremely active and eats great (in front of us) while the slider seems very sluggish, i have never seen it eat.  I did put 6 live guppies in the tank with them, and now 3 are left (5 days later) so maybe he ate one of those and that is why he doesnt eat any of the ReptoTreat gammarus I got. I also see the ridgeback laying on top of the slider on the smooth rock (which i large enough for both of them by far) under the UVB basking light, 25Watt that sit on top of the cage over over the top of the tank, about 12" away from the rock.  They are in a 20 gallon tank with a filter as well.  The turtle himself looks okay, shell is hard, swims around, climbs on the rock.
I guess my questions is if this is normal behavior for the slider? My son is already scared he is going to wake up and find "Gerald" dead and it would crush him.
But i did notice, when i picked him up to see if he was okay, would move if i touched him (which he did) i saw a very tiny little bug of some sort on the rock??? Should i be concerned. Thank you so much for your time.

Answer
Ridgeback? A Ridgeback Musk Turtle, or a Map Turtle (with a knobby back)?

My first guess is that the temps are too low. The water should be about 75-80F and the basking site should be about 90F.

Otherwise, I'd like ot point you at a great article: http://www.austinsturtlepage.com/Articles/newhatchling.htm and the general site of http://www.austinsturtlepage.com for more info and guidelines.