Where Did Egypt’s Priests Get Water To Turn Into Blood?

The very first of the ten plagues on ancient Egypt was the turning of the Nile river water into blood. But Egypt’s pagan priests did similarly with their magic arts. But this poses a seeming problem, where did the priests get water to turn into blood?
Alleged Contradiction:
The Bible says that for each miracle Moses and Aaron demonstrated the magicians did the same by
their secret arts. Then comes the following feat:
(a) Moses and Aaron converted all the available water into blood (Exodus 7:20-21)
(b) The magicians did the same (Exodus 7:22). This is impossible, since there would have been no water
left to convert into blood.