Any fishkeeper can tell you this, because like most animals, when fish get bored, they turn destructive. If your fish are tiny, like zebra danios, it's not a big deal. Larger fish, however, can dig up the substrate, move rocks around, shred plants, or otherwise make a pain of themselves. Also if you have more than one fish, they'll eat each other. Solitary fish may pick at themselves or stop moving much.
Fortunately, there are many ways to prevent fish boredom by providing a more natural and interesting environment. Rocks, tunnels, bubbles, plants, etc. help a lot.
You can also switch up their diet. Most fun I've ever had for $2 -- feeding live brine shrimp to angelfish. Their mouths can evert into a giant vortex of doom, sucking in hapless prey. They typically don't do that with flakes, just gulp them down. You have to give them live food to see how big that mouth can get.