Ps. Saulosi - concave stomaches, HELP!

My poor babies!!!

I'll admit that I have been working a lot lately and haven't been paying as much attention to my fish as I should (I still feed and change water regularly) but I haven't been looking at them much. I noticed recently too that I seemed to be missing a few fish, who have apparently died without a trace.

Anyways, I sit down tonight to look at my fish, and I notice they do not look well. Their bellies are concave and almost seem to be discolored.

They all still eat eagerly.

I searched and it seems that I should use metronidazole.

Can anyone chime in before I procede?

Update:

The fish are still eating like crazy and also behaving what seems to be normal. Their stomaches are looking much fuller than before and are about normal again. I have been changing the water every night, about 30%, and feeding 2x a day, soaking food in 2 cap fulls of metro and dumping the mixture, plus a little extra metro, into the tank.

However, their stomaches are still discolored, whiter than the rest of their body. Does anyone have any info about this? I'm hoping this goes away w/ the medication by the end of the week.