Sforno on Exodus 7:23/ספורנו על שמות ז׳:כ״ג

1ולא שת לבו גם לזאת, to recognise the difference between what G’d had done and what his sorcerers had done. G’d’s activity had produced a total change in the nature of the river Nile, a phenomenon which had been considered as inviolate, constant, incapable of being abolished. It had now been turned into “real blood,” so much so that all the fish had died. The changes effected by the tricks of the sorcerers were performed on phenomena that were unstable to begin with. Possibly, all the sorcerers produced was make believe, an illusion of blood.