I assumed bullfrogs didn't eat fish. I'd never seen any frog hunting underwater and they don't seem set up to be very good at it.
But I wasn't sure so I did some research. Many websites said they do eat fish. But to me these sites seemed to want to tell an interesting story more than convey info, so I kept going. Luckily there've been many studies done.
Here's one done on Vancouver Island. To me this was much more interesting.
The sample was about 5,075 American Bullfrogs over 5 years. 473 stomachs were empty and removed from most of the results, which to me doesn't seem right. I don't know what their thinking was exactly. But still 4,602 frogs is a lot.
2.8% of the frogs had fish in their stomachs. That's 129 frogs out of 4602. If you include the 473 empty stomachs, which I would, it drops to 2.5% of frogs.
Where it gets interesting (to me) is that it varied from site to site. At more than 70% of sites no frogs contained any type of fish. Not sure if this means fish were available at all sites or bullfrogs taught each other how to catch fish and will learn to speak in the future and ultimately hunt humans,
Planet of the Frogs. My assumption is there was some opportunist reason like water being shallow enough that fish were breaking the surface, but that's a pure guess on my part.
Clearly bullfrogs can eat fish. But to me "can" and "do" are a bit different and everyone would have their own opinion. I personally would say bullfrogs are not very good at eating fish, although possible. I doubt bullfrogs are chasing down fish, but maybe some future study would show they do. None of these studies showed how the fish are being caught.
For a backyard pond I would think it's even less likely a bullfrog would eat any fish and extremely unlikely they could eat all fish in a pond. But that's only my opinion.
(If he weren't venomous, he could have stayed, but... he is)