Thank you for getting back to me. I suspect that your turtles weren't healthy when you got them - that's often the case with pet store turtles. It sounds like you've provided good conditions for the most part. I have just a couple of suggestions to make them even better. A 20 gallon tank is a bit crowded for four turtles. It's usually recommended that you provide 10 gallons of tank per one baby turtle. If you don't have a good filtration system and make regular water changes, the water quality may not be very good. The bulbs in UVA/UVB lights are only effective for 6 months. After that, they may look fine, but are no longer putting out the proper rays. That could account for the shell symptoms you're seeing. Also, pet store personnel often sell lights claiming to be "full- spectrum" that provide everything reptiles need. Full-spectrum isn't the same as UVA/UVB. Rely on your own eyes when choosing a new light. A Reptiglo 10 is a good brand.
You have tried vitamin A, sulfa, and tetracycline. There really isn't much more you can do on your own. Your turtles are exhibiting signs of a severe respiratory infection. They need to see a reptile vet as soon as possible. Opening the mouth as if gasping is called "gaping," and the turtle does it in an attempt to get air into his lungs. Here is a link to a directory of reptile vets so you can find the nearest one:
You can't treat this at home, and without treatment, it is likely to turn into pneumonia. It may already have. Respiratory infections are the leading cause of death in turtles.
You probably won't be able to see a vet until tomorrow. In the meantime, there are things you can do to support your turtles. Adjust the basking light so the temperature under it will stay at 90*. Raise the water temperature to about 85*. The warmer temperature can give the immune system a boost. Be sure one area of the tank remains cooler so they can get out of the heat if they want to.
I hope you're able to quickly get your turtles to a vet so proper treatment can begin.
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