I don't want you to have to wait any longer so I'm going to go ahead and give you some information without knowing those additional things.
From what you have described, I suspect your turtle has a shell fungus.There are two ways to treat fungal infections. The most effective involves what is called 'dry-docking.' You'll need to prepare a 'hospital' for the affected turtle. A large plastic tote works well. It should be equipped with a basking area at 90*F and a UVB light. The turtle will be kept warm and dry, except for a 30 minute bath twice a day. The two baths are very important to prevent dehydration. 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). If the problem doesn't clear up within two weeks, you'll need to have your turtle examined by a reptile vet.
The second method is less likely to be successful, but is more convenient. You can use one of the sulfa bath products sold in pet stores. They are added to the tank. Besides being less effective, your healthy turtles will also be subjected to the treatment. With this method, too, a vet should be consulted if the fungus isn't gone within two weeks. If your turtle also has some early shell rot, the dry-dock method will help, but the sulfa bath will not. This link will take you to a directory of vets if you end up needing one:
Temperatures and other conditions that aren't quite right often lead to fungal infections. You definitely need to get a UVB light. You don't need a night light at all. I suspect you got your information on care from a pet store. Such information is usually wrong.Because there is so much misinformation available both online and from pet stores, I’m also including a care sheet I’ve written for painted turtles. It takes months to years before incorrect conditions result in health problems, so even a turtle that has seemed to do well, will eventually become ill. You can use the care sheet as a checklist to make sure everything is right. If you have more questions, let me know in a REPLY. I hope Art will be fine.
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PAINTED TURTLE CARE SHEET
Well-cared for painted turtles can live 30 years or more.
It's recommended that a baby have at least a 15 gallon tank. By the time the turtle is 3-4 years old, it will need a 60 gallon tank, so it's best to get the biggest you can in the beginning. You can also use a large RubberMaid tote. That's not as pretty as a tank, but costs a lot less. Set up the tank so there's a land area and a water area. Put the basking light at one end so the whole tank doesn’t get too hot. You want the water to be about twice as deep as the turtle is long. If the turtle is two inches long, you'll want four inches of water.
Temperatures and Basking Area
Turtles need certain types of lighting and need to be warm. Air and water that are not warm enough can lead to fungal and respiratory infections and unhealthy shells. 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 80*F, 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). If you live in an area that has farm stores, you can buy a metal light fixture made to keep baby chicks warm for just a few dollars. Don't buy the accompanying bulb, however. You need an ordinary incandescent bulb in the basking light. Hardware stores sell similar fixtures as work lights. The basking area should be kept at 88-92*F. Use a digital probe thermometer to be sure. You can adjust the temperature by raising or lowering the light fixture.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 in the low 70’s.
It's extremely important that you buy an additional light that produces UVB rays. A Reptisun 10.0 in the straight tube style is a good brand that does. If you choose another brand be absolutely certain it provides UVB rays. Don't take the word of pet store personnel, but read it for yourself. Full-spectrum, DayGlo, SunGlo, UV, or UVA are not the same thing. I'm putting a lot of emphasis on this because it's crucial to your turtle's health. Without this light, Metabolic Bone Disease (MBD) will develop because your turtle won't be able to produce vitamin D. Vitamin supplements are not a good replacement for the proper lighting. MBD causes a very slow and painful death.UVB bulbs must be replaced every six months as they lose their effectiveness after that, even though they may still look fine. Light that comes through a window isn't sufficient because the glass filters out most of the rays turtles need to stay healthy. To prevent MBD, turtles also need calcium. The easiest way to provide it is to place a cuttlebone in the tank. Cuttlebones are sold in bird departments of pet stores.
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. If the tank is too small, no filter can keep up with the amount of waste that turtles produce.
Feeding is an area where pet stores often give out bad information. Commercial food and animal products (meat, earthworms, canned cat food) should make up about 3/4 of the diet. The remaining 1/4 should be plant foods (dark lettuce like romaine, bits of strawberry or melon, etc.) Hatchlings should be fed every day. Older turtles should be fed 3 times per week. Overfeeding can lead to gout and kidney failure.