Hi Kevin. There is no problem feeding your tortoises cucumbers. They like them and they is a very good source of moisture for the tortoises. Cucumbers are about 95% water, but have a relatively high mineral content. The skin contains the cell salts and the vitamins. Hence they should not be peeled. Cukes are a source of potassium, sodium, magnesium, sulphur, silicon, chlorine and fluorine.