Hello and welcome. My name is ***** ***** I'm a biologist with a special interest in reptiles. I'm sorry to hear of this problem. It sounds like your turtles have shell infections.
The most effective treatment involves what is called 'dry-docking.' You'll need to prepare a 'hospital' for the turtles. A large plastic tote works well. It should be equipped with a basking area at 85*F to 90*F and a UVB light. The turtles 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 shells 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. Your local vet may be able to consult with a reptile vet by phone to provide additional treatment.
After the turtles recover, do make sure all conditions - temperatures, types of lighting, filtration etc.- are all optimal for their species. If you're not sure of what is best for them, let me know what species they are -sliders, painted, etc.- and I can help with that. For that, or if you have more questions, let me know in a REPLY. I hope the treatment I gave you above will cure the problem.
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