There's a difference between surviving and thriving. It's similar to putting a goldfish in a bowl. Some people are able to do this and kept a fish alive for a couple years, but there's no way that fish is happy and healthy. It's likewise with mixing frogs. They end up basically poisoning each other.
Also, right now you have store bought frogs mixed with wild caught frogs. If the frogs you bought at the store are captive bred, they could now be coming in contact with parasites they are not used to from the wild caught animals.
Again, I'm not sure why right now it's only the bullfrogs that are dying off, but I stand by my first post. I think the 10 gallon tank is overcrowded.
Does your city use chlorine or chloramine in the water? This usually affects fish pretty quick, but I assume it would affect the amphibians as well.