just want a quick check on something. if a fish limps and you raise, everyone folds, fish calls. you have isolated the fish. This is good. We want to play with fish.

But what about if the fish raises from any position (and we have position on him), do we play our normal range of hands vs an earlier raiser or do we change because its a fish - in particular do we raise a wider range than usual just to get to play them HU in position?

(never use these)

It really depends on their postflop tendencies and how they react to a 3bet. If they call a lot of 3bets, then you'll want to 3bet pretty light for value with hands which are ahead of their range, as you would love to get HU against the fish rather than allow other players into the pot. If your opponent makes a lot of postflop mistakes, in particular if they are too passive or fit-or-fold postflop, you can cold-call with quite a wide range of speculative hands. More competent players will barrel you off your equity often, but a fish might let you see free/cheap cards, yet pay you off when you hit.

You'd want to play a wider range vs the fish if he raises (and his raise does not signify a significantly stronger range). You'd love all your suited connectors and such if you're in BTN CO etc. One play actually is to isolate him further by min3betting or 3betting small or just 3betting normal sizing, even if you expect him to call very wide (or BECAUSE you expect him to call very wide) with hands like JTs QJs 89s, ATs, KQo, etc, because if other players on table are semi aware they will play super loose too and you want to be alone with the fish, even at expense of some more money in the pot. This is especially true if they will open light but not 4bet or get lots of money in easy (you can min3bet/fold easily). Obv in a spot where there are lots of super tough players they will take advantage of you and cold 4bet, but most of the times regs will not mess with regs when there's a spot on the table, or just overall aren't that high intensity at the low stakes so this is one thing you can do but you must be comfortable postflop, fairly deep, and, well the prementioned conditions must be true.

Depends what kind of fish.

Loose passive fish with stats like 32/3 should not be isolated with a 3-bet. Their raising range is so tight (JJ+, AK) that you should only call with pocket pairs and try and stack them with a set when they can't fold an overpair.

If the fish is more of a maniac, with stats like 32/22, then you can 3-bet a wide range, but be prepared for some wild variance, as sometimes he'll make a hand with JT or K9s, and you'll need good post-flop skills to avoid the traps. Checking the player's "fold to 3bet" stat should indicate whether you should call with hands that play well in position, or go for the re-raise expecting him to fold pre-flop.

Hm I think you should iso3bet even loose-passive fish whenever you can, they play so bad postflop. Its mainly just a 3bet for isolation.
If not be prepared to get squeezed the hell out of yourself, your range is so faceup weak when you flat a passive fish.
Surely the dynamic can develop that regulars know this and you will get 4bet a lot but its not that common, then you have to adjust again.