Thank you for your patience. I can tell that you care about your turtle, and want to take good care of him, but someone (probably pet store personnel, as that is common) has given you bad information on how to do that. If you want your turtle to be healthy and live a long life, there are some important changes you'll need to make. First, I'll give you a basic summary of aquatic turtle care. Then we’ll talk about your turtle’s health problems.
Your turtle must have a basking area. You may be able to set up the tank so there’s a land area and a water area if the floating dock no longer works for your turtle. The water in the tank should be about twice as deep as the turtle’s shell is long.
In order to be healthy, turtles need certain types of lighting and need to be warm. Air and water that are not warm enough can lead to bacterial and fungal infections. Turtles must have a basking area where they can get out of the water, dry off, and bask in very warm light. The ambient air temperature in the tank should be around 24*C , with the basking area warmer still. Over the basking area there should be some sort of lamp that will take a 40-60 watt incandescent bulb (or you can buy a ceramic light fixture made just for reptiles). The basking area should be kept at 29 to 32*C. Use a thermometer to be sure. You can adjust the temperature by raising or lowering the light fixture. Set up the basking area at one end - you don't want the entire tank to become too warm. A lack of a basking area also leads to infections. The lights that come with the covers on aquariums are not suitable for turtles. You'll also need a submersible aquarium heater that will keep the water 26 to 28*C. The temperature range of 20*C to 30*C is providing too much variation.
You'll need to have a 10% output UVB light. A Reptisun 10.0 is a good brand. UVB bulbs must be replaced every six months as they lose their effectiveness after that, even though they may still look fine. If yours is older than that, there is very little output at this point, and your turtle is no receiving any rays. UVB lights put out very little heat, so it’s essential that you also have a basking light.
Turtles are very sensitive to water quality. Even if you change the water every day, it can still contain harmful chemicals. A good filtration system is essential. Water changes are also needed even with a filter. Poor-quality water is another common cause of fungal and bacterial infections.
Feeding is another area where pet stores often give out bad information. Commercial food should make up only 1/4 of the diet. A complete type, such as Mazuri, should be used instead of one that contains just one type of food. Animal products (meat, earthworms, canned cat food) should make up another 1/4 of the diet. The remaining half should be plant foods (dark lettuce like romaine, bits of strawberry or melon, etc.). Your turtle also needs a plain calcium supplement. You can dust his meat foods with calcium powder, or provide him a cuttlebone (available in the bird dept. of pet stores) in his tank.You may want to look at these sites for additional information on care and feeding. I especially suggest that you check out the lists of appropriate foods.
Turtles do shed their skins, but it most often is a gradual process without the kind of peeling you’ve described over the shell and legs all at once. Fungal infections also cause peeling skin. The dark spot on the shell is most likely shell rot, which can be the result of no basking area. The most effective way to treat both conditions involves what is called 'dry-docking.' You'll need to prepare a 'hospital' for your turtle. A large plastic tote, such as Rubbermaid manufactures, works well. It should be equipped with a basking area at 29 to 32*C. and a UVB light. The turtle will be kept warm and dry. You'll remove the turtle from the water except for a 30 minute bath twice a day. Feed during the bath. After each soak, scrub the shell with a clean toothbrush with Betadine (available in drug stores) on it, but try not to get it on the skin, and especially avoid the eyes. Do this twice a day. Follow that with a coating of anti-fungal cream (the kind sold for women to use to treat yeast infections). The anti-fungal cream should be put on both the skin and the shell. If the problem doesn't clear up within two weeks, you'll need to have your turtle examined by a reptile vet. The following link will take you to a directory of reptile vets. I suspect that you live in the UK. If you’ll scroll to the bottom of the page, you’ll find the links for UK vets. Canada and Australia are also listed.
The turtle is probably eating his droppings because he is malnourished. An improved diet will help, but you shouldn’t feed him in his main tank. That results in dirty water and more health problems.
If you have further questions, just let me know by clicking on REPLY. I hope your turtle’s condition will improve quickly.