Moshe and Aharon did so, as Hashem had commanded, and he held the staff aloft and struck the water that was in the river in the presence of Pharaoh and in the presence of his servants, and all the water that was in the river changed to blood. And the fish that were in the river died...
It would seem that the reason the fish died during the plague of Blood, was simply due to the lack of oxygen in the river's waters when they turned into blood. What we need to understand is; if the fish died naturally when the water turned to blood, why did the Torah see it important to tell us about it, being that it was nothing more than a natural outcome?
We need to understand why indeed the fish died, as the Gemara in Megillah (יב ע"ב) says, במדה שאדם מודד בה מודדין לו - With the measure that a man measures, the Heavenly tribunal measures out to him, i.e. in the manner that one deals with others, Hashem will deal with him. Accordingly, because the Egyptians bestowed goodness upon the Jews by freely providing them with fish, as the Jews clearly acknowledged when they said, זכרנו את הדגה אשר נאכל במצרים חנם - We remember the fish that we would eat in Egypt for free, therefore, this merit in itself should have sufficed to prevent the fish from dying, so that the Egyptians should not bear a loss in that distinct entity with which they had bestowed goodness upon the Jews.
The answer seems to be that when the Jews said זכרנו את הדגה אשר נאכל במצרים חנם - We remember the fish that we would eat in Egypt for free, it doesn't mean 'free of charge' as the Gemara in Yuma explains, rather is referring to something else entirely, as the Midrash, quoted in Rashi, explains. If you would say that this means that the Egyptians would give them fish free of charge, this cannot be so, for has it not already said, 'Straw will not be given to you'; if the Egyptians would not give them straw free of charge, would they give them fish free of charge? What, then, did they mean when they said 'free'? They meant, 'free from any obligations to perform the commandments of the Torah'.
Nonetheless, it is still hard to understand why the fish died, for although the Jews might have not eaten the fish entirely free of charge, nevertheless, they definitely were nourished from those fish, as the Jews themselves proclaimed, 'We remember the fish that we would eat in Egypt'; therefore that in itself should have sufficed to keep the fish alive, so that the Jews should continue to have from where to eat and be nourished.
We can explain this phenomenon according to the Midrash (שמ"ר ,ט' ט') which expounds on it as follows. And the fish that were in the river died. The great scholar, R' Avin the Levite, said: The Jews became wealthy from the plague of Blood. How so? Picture an Egyptian and a Jew living together in one house, and the barrel there is full of water. When the Egyptian would go fill his ladle from inside the barrel, he would take it out full of blood, while the Jew is drinking water from inside the barrel. The Egyptian says to him, "Give me a little water from your own hand", but when the Jew gives it to him it becomes blood. The Egyptian then says to the Jew, "Let you and I drink from one bowl", and even so the Jew drinks water while the Egyptian drinks blood. However, when the Egyptian would purchase the water from the Jew with money, he would drink water. From these transactions the Jews became wealthy.
We need to grasp how Rav Avin was able to discern, from the fact that the fish died, that the Jews had become wealthy. We can explain that Rav Avin was bothered by this very question that we just posed; he found it hard to understand why the fish would die when that was the source of the Jew's livelihood. Consequently, he surmised that the Jews must have become rich from this very plague, and thus no longer had any need for the fish to sustain them.
We can now understand why the Torah felt it to be significant to tell us that the fish died. For we can say that, in truth, the water never actually turned into actual blood, rather it only appeared that way as a way to punish the Egyptians by making them be repulsed from drinking the water. Therefore, naturally speaking, the fish did not have to die. But nevertheless, they did die as punishment to the Egyptians. And as for the livelihood of the Jews? No need to worry, for the Jews all became wealthy during this plague, and no longer had any need for the fish.
זרע שמשון פרשת וארא אות ח
