I have two 100 gallon preform ponds. Every year the dozen of frogs dwindles to just one or two, and I suspect that the Black racer snakes are the culprits, as I saw one catch a frog last year. But this year suddenly all my Green and Leopard frogs vanished, and I don't think it was the snakes, as twice I saw a huge Bullfrog sitting by the pond, once at dusk, once at dawn. Last week I noticed it on the edge of one pond, and I snuck out there with a net. Bullfrog was gone when I got there but I saw the water ripples. After draining my pond I found the sucker hiding in the waterfall box. He/she was larger than a softball! It's been rehomed to a large pond about a mile away. And my remaining frogs are coming out of hiding.
Over the years I stopped buying goldfish for my ponds as every time they disappeared I found a fat Bullfrog. I introduced wild-caught fat head minnows to my ponds three years ago and they've been thriving. But I also discovered when I drained the pond that all the minnows were missing.
I hate Bullfrogs.