I'm not sure why exactly it would be only bullfrogs dying right now, but there are several mistakes it seems to me you're making. First, you're mixing frogs, and that's a really bad idea. They all release toxins that will hurt each other. Next, I wouldn't add a turtle with frogs, especially in that small of a tank. Turtles release waste in the water, and I think that in such a small tank with so many animals in it, that waste is going to build up pretty fast. I think in general, even if you only had the bullfrogs, you're overloading that tank.
What type of water are you using? It's not straight tap water, right? What about substrate? What are you feeding? What is the temperature and humidity in the tank?