I think that my Red-Eared Slider might be sick..

Question
I have had "Pico" for around 10 months now.  He lives in a small plastic aquarium-like container...I've read up on everything and know how his habitat should be.  He has enough water to swim in, but is also able to get out when he needs to.  I have a light and everything to keep the water at the right temperature.  The whole time I've had him he has been very active with a good healthy appetite (he eats a lot actually).  I examine him closely every few days, just to be on the safe side.  I keep his tank very clean.  However, lately, possibly the last 2 weeks or so, I have noticed that he will go to one corner of his tank and manage to position himself vertically with the cage, the bottom of his shell lying flat against the side of the tank.  I can't figure out why he is doing this, and at further examination, I noticed that this is taking a toll on his shell.  He is putting all the pressure on the tail of his shell, and the end of his shell is softer than the rest and starting to bend inward from the way he is sitting on it, so to speak.  I have noticed a crack in it, as well.  Is this normal?  Could something be wrong with him?  He's never done this until recently, and I am worried that it's not normal.  Should I take him to a vet?  Other than that, he is fine.  He eats normally and is active.  What should I do for him?

Answer
1. The words 'small plastic aquarium-like container' worries me, especially when it comes after the phrase 'I've read up...' One of the Golden Rules of healthy turtles is BIG spaces. We often recommend about 10 gallons of actual water per inch of shell length. Whenever possible, the sides of the tank should not be transparent.

Big tanks make life easier- more swimming space, so more natuiral behaviors. More exercise, less weight problem. No competition for food or resources, etc., etc., etc.

Transparent walls make turtles nervous- why can't they swim there? It bothers some more than others.

A little damage on the 'marginals' (the outer ring of scales on the shell) is annoying but not a health concern.

Another detail that worries me is the phrase 'he eats a lot'. A daily serving for a turtle is about the size of its head- it is really easy for these guys to overeat. You don't mention the actual diet, but it is also important that the food be very nourishing- and there is a lot of bad food in the stores.

A couple helpful websites might be:
http://www.austinsturtlepage.com and
http://www.redearslider.com

Good luck!