Did both small fish and big fish die in the plague of blood?
(7,21) And the fish (דגה) that were in the river died and the river became foul, and the Egyptians could not drink from the river.
The gemora in Nedarim 51b writes that R. Shimon ben Elazar taught that one who vows to not to eat דג is forbidden to eat large fish but permitted to eat small fish. And if he vows not to eat דגה he is forbidden to eat small fish but permitted to eat large fish. The gemora challenges this teaching from our posuk - "And the fish (דגה) that were in the river died " - did only the small fish die but not the large fish?
But why did the gemora bring its challenge from this later posuk which talks about what actually happened as a result of the plague of blood, and not from the earlier posuk where Moshe warned Pharaoh what would happen and said that "the fish (דגה) that are in the river shall die"? Since the gemora’s challenge was based on the simple logic that if the river turned to blood certainly both small and large fish would have died, as the Ran explains there in Nedarim, the gemora could have equally asked from here "Was Moshe warning that only the small fish would die?"
However, if we examine carefully the wording of the warning and the wording of what actually happened we see that in the warning it says that the water that was in the river "will turn to blood, and the Egyptians will weary themselves to drink water from the river". Notice that it does not say that "all" the water will turn to blood, and also that it says that "the Egyptians will weary themselves", which implies that they would in fact be able to drink the water, but only after much effort. But when discussing the plague itself it says "all the water that was in the river turned to blood…and the Egyptians could not drink water from the river". Here it says that "all the water" turned to blood, and also that they "could not drink", which implies that thay were not able to drink at all.
From this it seeems that the initial warning was that only the water which was close to the river's banks would turn to blood but not the water in the middle of the river. Thus they would still be able to draw water from the middle of the river, but it would require much effort and therefore it says they they will weary themselves. It follows therefore that if only the water close to river's banks turned to blood only the small fish would die since it is the way of small fiah to be found near the banks of a river, but the big fish would not die since they swim in the middle of the river.
Therefore, it was not possible for the gemora to ask its question from the earlier posuk, because according to the warning indeed only the small fish would die. But afterwards, since the Egyptians remained rebellious and did not listen to Moshe they deserved a greater punishment and all the water in the river turned to blood, even the water in the middle of the river, and thus they could not drink from the river at all. Therefore, from this later posuk the gemora rightfully challenges that if all the water turned to blood "did only the small fish die and not the big fish?" - surely not, since this defies simple logic as the Ran wrote.