Really, the question revolves around the word כל.
If we believe this term has always an absolute acceptation (meaning) this could trigger the paradox 'collen ndhlovu' enhances.
But, Are we obliged to apply an absolute acceptation (meaning) to the term כל, in this context? Not necessarily.
A parallel example will help us to understand. In Exo 9:25 this term is used in reference to the devastating effect of the hail (inside the divine 7th plague). We read, in fact, in Exo 9:25 the following:
ויך הברד בכל־ארץ מצרים את כל־אשׁר בשׂדה מאדם ועד־בהמה ואת כל־עשׂב השׂדה הכה הברד ואת־כל־עץ השׂדה שׁבר, "All over Egypt the hail struck down everything [כל] in the fields, man and beast, and the hail beat down everything [כל] growing in the fields and shattered all [כל] the trees in the fields." (New Jerusalem Bible).
It seems that the hail destroyed everything, with a meaning of כל of absolute acceptation... outwardly...
If we reach now Exo 10:5, we find that existed (after the divine hail) something the hail did not destroyed, and this concept is repeated three times in this verse: a יתר (a 'rest', or 'leavings'), הפלטה (a 'remnant', or 'scraps'), and הנשׁארת ('what remains'), 'from the hail'.
In a similar way, the passage cited by 'Perry Webb', that is, Exo 7:24, tells us that not all (in absolute acceptation) the water of Egypt was turned to blood. So, also in this case the term כל wasn't used with an absolute acceptation mood.
(Sorry, but the Keelan's cited 'wayyiqtol forms' got nothing to do with the argument debated)
The term כל has two acceptations, an absolute one, and a relative one (like in Exo 9:25).
If we understand this difference, the paradox vanishes...