Ah I won't have to eat shoes afterall. I figured this was a water chemistry issue. Sadly 0.25ppm Ammonia is bad news but 2ppm Nitrite is insanely toxic.

Honestly the water quality could have been out of whack for quite some time without proper testing. ACF are very tough animals, they can survive poor conditions for a very long time but eventually they will show signs of illness from the stress of being kept under subpar conditions.

I think you may just have too many frogs in that 20 gallon, not sure what filter is being used but it's overwhelming the biofiltration. ACF females are fat little things, they eat a lot and they poop a lot. They are messier than goldfish and you can't really keep 3 goldfish in a 20 gallon either. My advise would be to upgrade the tank to a 40B, the Petco sale I alluded to makes it a good time to upgrade.

I think with all three frogs in that 20 gallon you may wind up with the same situation down the road.