As i've said before, I generally try to be playing against the fishiest (ie worst) opponents I can find. What makes a fish? Well, its basically anyone who plays outside what is normally expected as winning play, and tends to donate their bankroll to the table. ie they lose over the long term at reasonably fast rate.
Generally fish can be classified based on their style of play, which can also be drawn out by their pokertracker statistics. In particular I like to look at their preflop statistics as a key starting point, because, basically if they're a poor player preflop, its also likely they are fairly poor post flop.
So in terms of pokertracker the important preflop stats are:
VPIP: The percentage of times an opponent voluntarily puts money in the pot (ie they limp, raise, or call)
PFR: The percentage of times an opponent raises preflop.
For 6max limit poker that I play, optimal stats are somewhere around the 30% vpip and 20% pfr (usually shown at 30/20 in my blog and generally described as a TAG - tight aggressive). More aggressive players (LAG - loose aggressive) can also be profitable playing a 50%/30% game or somewhere between that and the TAG statistics. I won't go into why those styles tend to be profitable, but if you pick up any 6max limit books, they shouldn't tell you much different from that.
In order of preference I like to be playing against:
1. A very loose passive - anyone who is seeing over about 50% of flops (VPIP 50), and raising say less than 5% I put in the loose passive category who I really want to be playing against. With luck I will even find someone with a VPIP of 80% which categorises them as a massive fish, and there's a good chance they'll give all their money away as they are playing almost any two cards, and likely are playing any two in the blinds.
2. The showdown monkey - This is actually a different statistic in pokertracker as it relates to post flop play. Optimal for a 30/20 player is to go to showdown (WTSD) around or just under 40% of the time. If I ever spot someone showing down 50% of the time or more, you know they are often chasing draws or cards they are not priced in to chase, or bluffing with complete air. These players are particularly profitable for me, as they will be calling big bets on the turn and river, and will often be drawing dead. Combine them with being loose/passive preflop, and they are the perfect fish.
3. A loose semi aggressive - The 60/20 type is also good to play against. You can use a normal preflop range for hand reading their preflop raises, but when they don't raise, you can pretty liberally raise them as they are often playing very poor hands, like unsuited connectors, gappers, Q2 - Q8 etc.
4. A very loose aggressive - sometimes you'll find a player who is showing something like 80/60. They are basically playing every hand, and raising most of the time. Any time you have a solid hand its worth 3 betting to isolate these players. Sure they will sometimes have a hand, and that may create some variance, but generally they will lose all their cash fairly quickly
It is also handy to have notes on how players play particular hands. Some will bet aggressively on the flop with 2nd or 3rd pair. Others will bluff any flop. With the right notes, someone who has solid statistics, can still be turned into a money spewing fish.
Who I try to avoid:
Basically I try to avoid the TAGs. I won't sit in a seat with a 30/20 player on my right. If the player to my right is a LAG running at 50/30, I will only sit down if he is terrible post flop, preferably showing down too much and showing as a big loser in my Pokertracker database.
As per my seat selection post, I dont really care if there are TAGs on my left if there are fish on my right. Sure they'll probably take some money from me, but not as much as I'm making from the fish.
What it means to be playing against fish:
What is means, and what you need to be prepared for, are the bad beats. They are going to be playing rubbish cards. They are going to be chasing their 4, 3,2 or 1 outers. They are going to hit them. Not often enough to not be a complete loser, but often enough to annoy you and potentially put you off your game if they hit a few in a row.
In my last session alone I had AA cracked the three times I received it. Once by someone with T5s who flopped TT5, which also rivered me the backdoor nut flush with four spades on the board. That hand alone cost a lot. Another crack was a guy playing 85s who fopped a set of 8s, and finally the last guy had A6s on a 234 flop, with a rivered 5 which gave me the bottom end of the straight only to lose to his 6.
If you're playing only against TAGs, those beats basically won't happen. However, nor will they keep playing T5s for the 90% of the time that they don't hit anything, or hit something but are still behind giving you an even bigger pot.
Current bankroll: $25,450
April time played: 12.2h
April hands played: 2,875
April profit / (loss): $750