Thank you for getting back to me, Kim. I suspect that you got your information on care from a pet store. While you would think you could get correct information from them, you cannot. The staff may mean well, but they are seldom informed on proper care. Turtles are more difficult to care for than most people would expect. They also can be expensive to get started with. They can live 30 years or more, when properly cared for. I can tell that you care about your turtles and want them to be healthy, but in order for that to happen, you'll need to make some important changes. It's certainly not your fault that things aren't quite right - you've just been doing what you were told. Weaker turtles are often affected first by conditions that aren't right. While your other turtle seems to be healthy and growing, it's only a matter of time before he develops problems, too.
From your description, I believe your turtles are yellow-bellied sliders. Silver dollar turtle is just a cute name pet stores have made up to sell more turtles. It's recommended that a baby slider 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. Two turtles will need a 90 gallon tank. 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. 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.
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 fungal and respiratory 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 75 (24*C) degrees, 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 85-90 degrees (29 to 32*C). Use a thermometer to be sure. You can adjust the temperature by raising or lowering the light fixture. If your tank is small, set up the basking area at one end - you don't want the entire tank to become too warm. 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 78-82 degrees (26 to 28*C).
It's extremely important that you buy an additional light that produces UVB rays. A Reptisun 10 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, UV, UVA, Dayglo, and daylight 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 turtles 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.
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.
Feeding is another area where pet stores often give out bad information. Commercial food should make up only 1/4 of the diet. Animal products (cooked meat, earthworms, canned cat food) should make up another 1/4. The remaining half should be plant foods (dark lettuce like romaine, bits of strawberry or melon, etc.). Food should be dusted in a plain calcium powder (no vitamin D3 in it). Flukers makes a plain powder that is inexpensive.
You may want to look at these sites for additional information on care and feeding.
Sometimes, people end up with more than they bargained for with pet turtles. with today's economy, some can't afford all the needed equipment. If all this sounds like too much for your circumstances, give me your state, and I can refer you to reptile rescue groups who would take the turtles and care for them. I hope that isn't your situation, but I wanted you to know there are options if you need them.
As for sexing, until the turtles reach sexual maturity, it's very difficult to tell the sex of slider turtles. In mature turtles, the males have longer claws and thicker tails. Females are usually much larger than males. The following site has photos of males and female sliders that you can compare to your turtle. You'll have to scroll down to the proper section of the page.
If you have further questions, or want a list of rescues, just let me know by clicking on REPLY. There's no additional fee for such follow-up questions, and I'm happy to help. I hope you enjoy many years with your turtles.