African Dwarf Frog Curled Up at Bottom of Tank?

Anyone out their if you guys could tell me what is going on. I thought the water might be too cold but got a thermometer and its perfect. I thought he might have fungus on his back but its such a small dot not sure yet. My frog is curled up at the bottom of the tank acting freezing. The water temperature is 78 degrees. Ammonia levels are fine. Not sure of the Nitrite or Nitrate levels. What could this be a sign of, he's 9 years old. He still eats like a cow when I force him to eat but the rest of the time he just sits at the bottom of the tank curled up. He responds if you tap him though. Thanks!