Your turtles actions are certainly because of the chlorine, and a dechlorinator is absolutely necessary. I don't recommend the dechlorinators with a slime coat additive because it's one more chemical the turtles are exposed to, and one more you are paying for that is not needed.
The professional standard is Amquel, or try this generic product...
http://www.aquaticeco.com/index.cfm/fuseaction/product.detail/iid/5685/cid/1597
Safe for use on food fish and shellfish. This highly concentrated powder completely neutralizes ammonia, chlorine and chloramines from fresh and saltwater. It is non-toxic to fish and invertebrates. Safe for use on food fish and shellfish intended for human consumption. Dosage rate: 1 oz ChlorAm-X® per 235 gallons of water removes 1 mg/L of total ammonia. A 5 lb box treats 18,749 gallons.
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"It isn't pollution that's harming the environment. It's the impurities in our air and water that are doing it."
Governor George W. Bush, Jr.
"Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us."
Calvin and Hobbes (Scientific Progress Goes 'Boink', 1991)