Specifically, what type of UVB do you have? Is it a bulb
or a long fluorescent tube like for fish?
I would put your lights on a timer to come on at 7am and
off at 7pm. It is best to try and duplicate nature. 18 hours
of light is not normal for these guys.
The reptomin sticks are a good staple food. As your turtle
grows you need to increase variety.
Please take your turtle to a qualified vet. My bets are
on some type of metabolic bone disease which if left untreated
will kill your turtle in a long painful death.
Please let me know ASAP what type of UVB source you have.