Well, I suspect that this baby is being picked on by one or
all of the others. Are they all in the same enclosure and
if so how big is the enclosure? Are the other turtles the
same size as this baby? All turtles kept together should
be very close in size.
Try putting this one in its own enclosure for a while complete
with the basking area and lights. Give it some time and see
if its behavior changes. I went through this myself with
my babies and this worked. Eventually I was able to reunite
them.
Others have definite opinions about temps but I myself
think a constant 80 degree water temp is too warm. In the
wild they have access to many different temperatures and
can choose freely which they like best. I keep my hatchling
Painted turtle's water from 72 in the cool end to about 76 in
the warmer end and the basking temp is about 85. He is
active and eats very well.
Let us know the outcome but I really suspect he is being
picked on.
Linda