This is completely normal. Simple fact is, fish is a large part of wild slider diet - and really should be in captivity too. Maybe at first he just wasn't hungry... or was intimidated by them... or just didn't feel like chasing them... could be any number of reasons, but no matter how you look at it, any kind of fish are just swimming snacks to almost any aquatic turtle.
People ask me all the time if they can keep fish with their turtles, and I always say the same thing: You -can-, just don't keep them with any fish you will be terribly upset to lose, because it could happen overnight, or it could take years, but eventually the fish will become a meal. I had three large goldfish in my musk turtle tank for over a year. The turtles never bothered them, then one day... poof, no more fish.

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The lips I never kissed, are lips I can't forget
The dress you never wore, on the day we never met
The name I never whispered, as it echoed in my mind
The man I never was, the sweet thing that's not mine.