What will agitate the fish?
Radak #1: This is an exaggeration; the noise and panic will be only in Eretz Yisrael. They will be so great, that it is as if the fish, birds, Chayos and vermin shake.
Radak #2: The place of the sea in Eretz Yisrael will shake. Similarly, birds and Chayos [in the place of the Ra'ash will shake].
Malbim: Yes. The shaking will spread to man and animate forms ? beings that swim, fly, live on the land, and creep. Also the inanimate will be stricken ? the mountains will be destroyed.
Will the mountains be destroyed?
Radak: Yes - "v'Nivka Har ha'Zeisim" (Zecharyah 14:4). Perhaps the same will occur to other mountains.
What are "ha'Madregos"?
Rashi #1: They are erect rocks that are suspended; they appear to be falling.
Rashi #2: They are mounds of towers. They dig around them and throw earth in the middle to raise the mound. They are made like steps, so the earth will stand. After it is compressed well, they remove the frame that holds up the step.
Radak citing Targum Yonasan: They are towers (they will be uprooted).