no insult intended, but it sounds like you need some of the basic knowledge of how to care for your RES...so I'll begin there, leaving out some details you will find if you read the other messages on here.
ok...here goes:
You need a UVB light source to help the turtle have a healthy shell. You need a basking area so the turtle will dry its shell while warming itself up. Basking area should be 85-high 90's. I keep mine about 95 and they love it. Water can be cooler, but too warm and they won't get out to bask, too cold and they can get sick. I keep my water about 76-78. Different turtles seem to like slightly different temps. If you set your temps like this and they don't bask, make the water cooler to encourage them to get out and warm up.
Diet: you need to vary the diet more. Live fish are ok, but as a RES gets older they eate nearly exclusively vegetables. Anacharis is a popular food. They can also eat corn, carrots (cooked) and many other things.
Water depth: don't worry about the turtle being small. As babies in the wild, they swim in water many feet deep (ponds and lakes). Put all the water you can in the tank that you can without having it so high it can get out from its basking spot.
It sounds like you have split the tank and tried to provide "land" or wooded area for the turtle. Don't give this the same priority as the water area. They are aquatic turtles and only come out to lay eggs or bask. They really only need enough area to get out of the water and warm up.
Reading up on all the info available here and at Turtletimes.com is a great start for information. I have read about so many problems people have had and the fixes that I have been able to avoid ALL problems with mine. Thanks to these sources, I started on the right foot with my turtles and havn't had even the first medical problem or sickness. If you can do the same, you will find it much less stressful and cheaper than if you chose to skimp on equipment and wind up paying big money for medical bills or medicines.