Bearing in mind that all the water in Egypt turned into blood, which water did the magicians use to copy Aharon's act?
Targum Yonasan: They performed their act with water that they obtained from Goshen, where Yisrael lived and which was therefore not affected by the plague.
Hadar Zekenim (1 - verse 17, citing Bechor Shor): After the water turned to blood, it returned to be water again. (Refer to 7:21:151:1.)
Da'as Zekenim (1), Hadar Zekenim 1 (2): They brought water from elsewhere.
Da'as Zekenim (2), Hadar Zekenim (3): They dug for water. 2
Da'as Zekenim and Hadar Zekenim reject this, for it says only below that they dug for water. We cannot say that the Torah is out of order within one Parshah, unless it is proven! (Perhaps now, only the magicians dug for water, and when people saw that such water was drinkable, "all Egypt" did so! - PF)
What exactly did the magicians do?
Seforno (Pasuk 18) 1 : They were able to make the water resemble blood in looks only, but not to actually transform it into blood.
On what excuse was Pharaoh's refusal based?
Rashi: Placing Moshe and Aharon on a par with his magicians, who copied Aharon and turned water into blood, 1 he accused them of bringing straw to Ofrayim (a town where straw was to be found in abundance) - seeing that witchcraft was rampant in Egypt.