In reality, the situation is a bit more complicated than it seems at first glance. Yes, Callagur borneoensis is sometimes mentioned as a relatively peaceful option, but even it is not “guaranteed safe” for coexistence with fish in the long term. In most cases, the problem is not aggression per se, but instincts: any fish that fits in the mouth or behaves slowly is perceived as potential food. Therefore, even “peaceful” species can eventually start hunting.
More complicated? Hmmm...maybe...but I almost think that it would be more accurate to say that the situation is actually not complicated at all, but rather very simple.
So, to put it simply...it's impossible to say with certainty that this turtle or that turtle will or will not live with fish. Sometimes it works...sometimes it doesn't. It's literally as simple as that. Sure, there are some species that are almost guaranteed to be dangerously predatory and/or aggressive...softshells leap to my mind here as being dependably unsuitable...and virtually any species will include individuals that might be problematic. I only have a single turtle now, but I have kept many more in the past and have had a lot that were completely inoffensive to most fish. I'm talking about individuals that lived with fish, with no predation, for years. Of course, there have been at least as many that were
mostly safe...but not entirely trustworthy.
I think a huge majority of the cases where a turtle was found devouring a fish started with the turtle finding an already dead fish and thinking "Well, dang...it's already dead, so..."
My hobby has been downsized significantly in recent years, but my current little Stinkpot has lived with an assortment of fish since I got him 9 years ago as a hatchling. He will eat small frozen/thawed fish if he's hungry enough, but it's certainly not his favourite food. He much prefers snails, worms, shrimp and vegetation, and his main diet is commercial pellets. He is one of the few turtles I've owned which I would unhesitatingly keep with any fish without concern for their well-being. His only transgression is with plecos; he certainly won't nibble on them, but he will attempt to mate with big ones!
Most of the other fish-friendly turtles I've kept...to include Painted, Spotted, Red-ear, Map, Diamondback, Blanding's, and a few exotics like Reeve's, Ornate Wood, etc... seemed just a bit untrustworthy, enough so that I wouldn't put valuable or hard-to-replace fish with them. But, frankly, I've never had a turtle that was fine with fish for an extended period, but who then suddenly "snapped" and became a problem...as many cichlid species are known to do. IMHO if a turtle is going to be a problem, you will know it right away.
Instinct? Obviously an important motivator. But instinct should have captive turtles running and hiding when we approach them. When...not if...they do the exact opposite, they're displaying a learned behaviour. Many...probably not
most, but
many...seem to learn not to bother fish.
But, as with most things in life...you pays your money and you takes your chances.
Edited to add: Dang! Necropost! I hate when that happens...