Shoal Rush does not do knock down. Shoal Rush makes Tide Pool go into Frenzy, which increases the chance for Tide Pool to knock down.
But like ALL knock downs in the game, if it happens at the same time as Whitecap or as a knock down from the Commando, then they can stack.
But Shoal Rush itself does not do knock down, so far as I know. I don't THINK Tide Pool can stack with itself. So I suspect you're either seeing it from Commando or Whitecap stacking with Tide Pool.
This is the combo I've been playing recently and I do see things go flying from time to time. Not with all that much frequency though. And it all dies so fast, I don't really care.