White spots are not evident on your pictures. If you manually peel off the loose scutes, this may cause damage and "white spots" on a young slider. More likely, these may be due to calcium (hard water) effusion under unshed scutes (see this all the time). Are you in a hard water area?
RE your sandfish: keep an area of your tank at over 100 degrees F by using an overhead incandescent light with a reflector. Use the "Calci-sand" (high calcium sand available in other brands at your pet super store). My Arabian sandfish (A different species) have done well for 8 years under these conditions. I feed a simple diet of crickets and mealworms. These are totally delightful creatures and soon learn to "beg' for food when you enter the room.

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