Sick African Dwarf frog

This morning I look in my tank and everyone is fine. Now I come home from church and I have a dead frog. So now I only have two african dwarf frogs left in the tank. I leave the house again and two hours later come back to see that I have one frog who is barely moving and hardly responding to anything. The frogs are fed flake foods and frog & tadpole pellets. They live in a 10g with a heater and the temperature is set to 78F. There is also a bubbler in the tank. I have three zebra danios, two swordtails and three cory cats in the tank. I just freshly cleaned the tank about three weeks ago. This is not my first time owning frogs, but this time I am at a loss as to what could possibly be going on with these little guys. Is there any way to treat him? He is barely moving and I don't know what to do if there is anything I can do. Thanks for the help...

Now both my frogs seem like they are doing fine. I don't know what's up with them. Last time I checked the ammonia it showed as being fine.

I clean the stones and gunk (ew yucky stuff..haha) every month. I do different size water changes throughout the months. These are brand new frogs that I purchased about two weeks ago.

Currently the one frog is bloated looking and is floating around the top. He is not dead because he tries to move away from the top but he cannot. When I take him out of the water and put him in a cup of fresh new water he becomes normal almost so it seems...

Okay now i only have one frog left that is alive.