When i started out with fish i had some african cichlids in an uncycled tank, two died they didn't have an indented stomach, they just had little coloring, were inactive, and scratched them selves by rubbingg up againdt decor
Other uncycled tank symptons include shimying(swimming on the spot), and slightly bulged eyes not to the extent of popeyes, also most of your fish should have dificulty try to get the test kit asap i think
But this sounds like a diet problem to me though i'm by far not an expert