When Moses turned the water in Egypt to blood, the Pharaoh’s court magician duplicated the act. ......Where did he get clean water to do that, if all the water had already turned to blood? ....
Targum Yonatan explains that the magicians turned some of the water in Goshen into blood. R' Avrahan Ibn Ezra and the Baalei Tosafot write that the Egyptians dug new wells (see the following verses) and the magicians changed the water in these to blood. This explanation is interesting in the fact that the Egyptians were willing to afflict themselves with additional suffering; the main thing was to oppose Moses and Israel.