Common names can be a bit deceiving, but I am assuming that your 'dwarf Congo frogs' are Hymenochirus sp. Now although these frogs can get bloated for a number of reasons, including certain diet and bacterial infections, I would wonder if you may have originally had more guppy fry than just six, and the larger frog helped thin them down during the night when the fry would have been likely resting near or on the bottom.
Incidentally, that is a rather smallish aquarium for that bioload....what sort of filtration and media are you using, and how often do you do water changes and what percentage do you do?
Incidentally, that is a rather smallish aquarium for that bioload....what sort of filtration and media are you using, and how often do you do water changes and what percentage do you do?