There are many species of fish.
Categories, and sub-categories....
These are all the passive types of fish I can think of at the moment. Watch all your opponents closely, and you will see how they play. Recognize first if they ever raise pre. That's the first clue you will have that they are a fish. Limping. If they are passive, divide them into loose/tight/station/fit or fold.....
Loose passive: General term. This describes the people who never raise (very very rarely anyways), they never do anything but limp and call. They play probably 75%+ range. Like when these guys fold, not even limp, I will wonder to myself what they could possibly fold, considering some of the hands I have seen them show down.
This first one is the type of player the OP is talking about....
Loose passive station: These guys will call flop and turn with any pair, inside straight draw, one over card to the board (think A-X, calls flop and turn, river A). These players are just about the closest thing to a walking human ATM machine. They will readily dispense cash. They just hate money.
Stategy to beat this player - Isolate them pre flop with a good hand. One that dominates their hand. Make a hand, and value bet, relentlessly. Don't even cbet bluff them, if they never fold to cbet. Make hands, bet them. Simple. Raise pre, make hand, punish fish. Repeat. Don't try to bluff a calling station. Just don't waste money. They don't fold, you know they don't fold, so don't bluff them. Don't put a single chip in the pot postflop, if you don't have at least a big draw. They don't fold dude. I feel like I have covered that enough now hopefully.
IF this player ever raises you, or gets aggressive, they will almost always have 2p+. If this player raise pre, or 3b pre, yeah, they just have the nuts, or near nuts. Even an open from a guy like this is probably going to be super duper narrow range. Recognize how passive they are, and take any aggressive action seriously. They aren't C/R you like a sicko with A-10 on A-9-3r, and your A-K is good. It's not good. If they C/R you on that board, and you have A-K, yeah dude no good. You can value bet them relentlessly with TPGK, but if they show aggression, yeah you need to really think "what do I beat?"
The Station is a good target for offensive over bets.
Loose passive fit or fold: These folks just like to see the flop. Loose. Wide range. They are looking for that miracle flop for their Q-4 "suited", that they limp UTG, and call a huge ISO raise. They fold if the miss the flop. So yeah, they fold alot.
How to beat them - Raise their limps with a wider range, always try to be in position. Cbet tons of flops. Just watch these people, they see flop, they fold, see flop, fold. If they don't fold to cbet, they got something finally. If they C/R, or any aggro action post flop, it's 2p+ usually. So if called when you cbet, proceed on turn/river with caution. They might be on a draw perhaps, but you better be sure in a read to double barrel. They usually don't give up after flop call easily.
Weak passive: This player limps all hands except maybe 10-10+ and A-K, mayyyybe A-Q if they are feeling frisky. They generally will call raises, and if you raise them too wide, their limping range may be ahead of what you raise them with. They generally play tight, but passive. They usually won't lay down top pair, like ever.
How to beat them: This type of player just folds way too much post flop. So, you put pressure on them. However, the key is to realize when they have a made hand, and back off the gas. Cause if they made TP, or they have a strong draw, they aren't folding. So, you pay attention, watch what they do, and only empty the clip on them when you know you have a better hand. Basic poker 101. Don't value own yourself against these guys. Don't go triple barreling on of these dudes with TP meh kicker. They fold alot. If they don't fold, don't get too invested with out a strong hand.
These are just some thoughts on the subject. I play all live poker, and I play 1/2 and 2/5. That's where my experience comes from. I'm sure there's fish like these everywhere? Online? I dunno.