Why are fish so fast?

Not (directly) related to Potomac Creek, but I was watching fish in my two aquariums this morning and started wondering just how the devil these creatures are able to dart around so quickly. When they're spooked my eyes can barely follow them. So, I googled "How can fish swim so fast?" and hit upon this wonderfully lucid explanation. I thought I'd share it here. 

The man's English is fantastic, but his accent kept me worrying that I'll miss something important. So I suggest turning on closed captioning (cc).

Enjoy!

Source: Internet Archive Book Images - https://www.flickr.com/photos/internetarchivebookimages/20483737440/Source book page: https://archive.org/stream/completeaquarium00inne/#page/n264/mode/1up