Fish are weak, unknowledgeable opponents that require a different strategy than knowledgeable opponents.
They’re the source of most of your profit, so you must locate them, get in their game and preferably put them on your right. Many players fear getting into games with great players, but they are approaching the “game selection” calculation from the wrong perspective. Assuming you play reasonably well, the good players will take little in edge from you, but a fish will likely over-compensate for any edge loss in a big way by providing a large edge in numerous situations.
Once you have him in your game, you need to size up his tendencies. Is he a passive fish, an aggressive fish, is he intimidated by the game and folding too much, or is he an uncertain soul that is paying everybody off? Once you determine his tendencies, determine what counter-strategies to apply. Do you want to bluff more, bet more, bet larger, etc? Think about how each fish’s weaknesses can be exploited.
Because your edge is larger when fish are in the pot, you can widen your range against the fish. That said, don’t overcompensate and become a fish yourself. Keep in mind the other players in the pot and those yet to act. You don’t want to put yourself in significant negative equity situations to our other opponents.
Most importantly, don’t tap the aquarium, it scares the fish. I constantly see players belittle poor play. It’s no way to treat your customers!! It’s both bad for business and classless manners, also.