50nl
UTG folds
MP folds
CO (95% fish) opens $1.5
BU ( hero) 3bets to $5.25 100bb eff w/ AdAh
SB folds
BB folds

flop pot $11.25 9d 6h 5c
Co checks
Bu hero checks

turn pot $11.25 8s
co checks
bu hero checks

river pot $11.25 6c
Co checks
Hero bets $8.5
Co check raise to $39 leaving behind 3.75
Hero needs about 33% to call

Flop I decided to check , thinking it could be higher ev at mixed frequency. Fish is unknown , but I believe a lot of fish would make a lot more mistakes vs check by overstabbing turn vs a capped range, due to his range having a lot of air/bad hands . By betting 2/3rd pot on flop , a fish will fold a lot of hands that he might make mistakes with and I don't see a lot of worse hands calling for 3 streets , only maybe some 9s on certain run outs, 10-QQ, which are probably not a large portion of his range . By checking , it allows him to hit a pair / over bluff .

On turn , I think standard check.

On river , I'm lost on what each kind of fish does, mainly the passive fish.

Do passive fish , look to check raise or do they immediately seek to get value from A high hands / pairs? I could see that fish don't see me calling A high often, which is a large portion of my perceived range and could check raise with his sets ( don't think passive fish would check a lot of 7's on turn,if any) when I have a 6 or checked back overpairs. Passive fish tend to slow play alot at a high frequency right? Or their frequency is mixed , but leaning towards trying to get value from sets on the river ? Leaving their value check raise region being close to 3-4 combos of sets/quads

Vs Spewy/aggro fish , i could see this as being overbluffed by alot . The spewy fish i think would prefer to bet river ,even turn, more frequently w/ his sets , two pairs, 7s to get value(7's probably bet all the time during turn). Which leaves his value check raise range very very narrow and, with his spewy nature, could easily overbluff.

Vs whale (this player unlikely a whale) , slightly unsure , aggro whales similiar to spewy/aggro fish, but would do it more at an extreme. Vs big station whale I think a lot of them don't bluff their stack at all in this line and might even slow play more.

Is this a close decision? or is this likely a call , since all kinds of fish , bet everything for value on the river.