I have a 3 year old Yellow Bellied Slider Turtle, I was wondering what size tank would be correct. Also I was told I was feeding it too much as it was getting a little chubby but now it gets fed every other day is this right it has a mixed diet of fruit veg and turtle food but I don't give it water as it dosnt drink it? Other than that I have a water filter and heater I am replacing its uvb lighting and just basically wanted to know if I'm doing everything right.
This is a large turtle, and will require a large enclosure when grown, so the bigger the better. I don't recommend a glass tank, as it will become nearly impossible to clean it as often as will be required. I do recommend planning for your pet's future, and considering building an outdoor enclosure for summer use, with a in-ground pond. The natural sun and access to growing plants and wild insect prey will be extremely beneficial for the turtle's health.
A plastic enclosure such as a Waterland tub would be most appropriate, but for general size, a 100 gallon aquarium is what your turtle will grow into. Use an over-sized filtration system, and change the water if it develops any cloudiness, or an ammonia odor. (Many turtle owners underestimate how often the water should be changed). Disinfect the enclosure regularly as well, using a bleach solution left on for 5 minutes, and rinsed very carefully to remove all bleach odor, before re-filling and returning the turtle to it.
Generally speaking, fruit should not be part of a slider's diet, and this may be why your turtle became overweight...though I do wonder who diagnosed it as being overweight. (inability to retract all limbs properly into the shell would be a sign of obesity). This turtle is very overweight: http://img31.photobucket.com/albums/v93/PetJunction/ellasleg1.jpg
Here's some good advice on how to tell: http://www.turtleforum.com/forum/upload/index.php?showtopic=130425
I found it odd that you mentioned not giving your aquatic turtle water... of course it drinks the water in its enclosure (which is why it must be kept very clean).
I think your turtle's diet could be improved. Better advice can be found here: http://www.austinsturtlepage.com/Care/cs-yellowbelly.htm
Vegetables for turtles would include chopped greens such as arugula, collards, dandelion greens, and turnip tops, and aquatic plants such as anacharis and hornwort. Fresh foods include live insects and mealworms, and chopped fresh tilapia (not frozen or previously frozen). Pellets should make up only around 30% of the diet. Dust fresh foods with a quality calcium supplement containing vitamin D3. Use a thermometer with a remote probe to ensure your turtle's basking platform is at 90F at turtle level, and that the turtle can haul out easily and get completely dry there.