In case you didn't know, "fish" in the poker world is a derogatory term referring to inexperienced and/or poor poker players. If you call someone a fish you aren't paying them a compliment, which brings us to the first topic...
Don't call people fish.
There are several reasons for this, not the least of which is the fact that it is rude. You might think that complaining and berating other people's bad play is limited to the semi-anonymous land or internet poker but it happens just as frequently in brick and mortar cardrooms. Besides being rude there are some other very good reasons not to do this:
It is misplaced hostility
When someone catches a miracle card to beat you on the river you aren't really angry at them--you are angry at dumb luck. In reality you couldn't be happier with their play. It is a natural human desire to spread the misery but it is especially misplaced in poker.
It often causes them to play better and/or more aggressively
Keep in mind that the best table environment to play the strategy on this site is a loose passive table. That means a table where people play any two cards, don't raise very often, and call with slim hopes! Why would you want to shift someone from playing exactly like that to someone who plays "good" cards and is aggressive with them?
It can affect the rest of the table
Even worse than making your single opponent play better you've alerted the table to a few things. First that they are not playing very well--many of them already know that but they want to have some fun. Secondly that you are not there to have fun, and thirdly that you are now probably at least a little bit on tilt--a fact they will take advantage of.
The fish might not be that fishy.
Don't form an unchangeable impression of a player based on one hand. Did they raise prefop with 76 suited and then crush your Ace Queen offsuit when they completed a draw on the river? Were you deceived about what they were holding? What may seem weak in the context of a single hand may in fact be an integral part of a strong overall strategy. Be aware!
Also keep in mind that with lots of people calling that lots of marginal drawing hands are either correct or "almost correct." Were they really that out of line with the draw? Calculate their pot odds and see. You might be surprised how often they aren't.
Fish are powerful
The combination of draws that exist when you have a lot of callers can be overwhelming. Really it shouldn't be surprising in the least to have your set/two pair/top pair ace kicker beaten on the river. After all taken as a group you are often a serious underdog to win any particular hand. Maybe instead of feeling cheated when you lose a hand you should feel quite lucky when you win one.
You are the fish sometimes too!
Playing lots of suited connectors, suited aces and just play old connectors as you'll often be doing against a loose passive table means that you will often be the person sucking out against the best hand. This is profitable because of all the other callers in the hand... Surely you wouldn't want to play a heads up match where you had the nut flush draw and your opponent had top two pair.
(Some) Players just want to have fun.
For some people playing poker is a social event. They drink, they joke around. No they don't play optimally and yes they are long term "losers" but for them it is entertainment expense. Is there anything wrong with this? Of course not. Not everyone has to play poker to make money--it can be a very fun game to play. Why ruin their time? Be a good poker citizen and joke along with them and make the game fun for them and that includes when they drag the monster pot with 7 2 offsuit. Roll your eyes, sure, but be jovial and let them have their 15 minutes in the sun.
Just something to think about.