Gray tree frogs and toads aren't going to stick around the pond longer than for breeding. I'd look into getting some tadpoles, northern green frogs or northern leopard frogs, that seems to be what I find most consistently in the pond behind our house. I personally would not introduce bullfrogs to the area if they aren't already there. I've noticed a huge decline in the frog population in my neighborhood since I noticed bullfrogs for the first time about 4 years ago, I can only imagine it's going to get worse with them eating everything.