Sunken belly can be a result of internal parasites, vitamin/nutritional deficiency, and TB /Fish Tuberculosis.
Anyone have other known causes?
Numerous times, I have had Guppies dies with similar condition die. Lower abdomen/stomach area increasingly hollows and then death.
earhtmother - you had mentioned (on a different thread) this happening after a, Guppy specifically, gave birth. I
This happened to me one time, I remember a fish dying after giving birth and died and the belly looked hollow. within a week later, might have been a few days.
Earlier on I had not seen this type if illness in my tanks.
After a few rounds of losing fish like that, I realized it was likely to be internal parasites.
This was cemented by a group of special fish which came from a serious Guppy breeder and "should" have been fine. and was not and lost the trio within a month or 2.
Then I began treating all new fish with a week of deworming fish flake food. The medication was Fenbendozole.
After that, my new fish additions seemed to be much more vigorous and healthy.
At some point year before last year, I didn't have the medication anymore and failed to buy more as my original supplier was out of stock for an extended period of time.
May also mention that the liquid medication was solidly rejected by numerous attempts with different fish.
Since for a period of time back when I ran out, I wasn't really buying many new fish it sort of slipped by.
Then more recently I could not get it near me - checked all around the stores who recommended the liquid and soaking food which was in my mind a waste of time trying to get fish, not too interested in eating to eat weird yucky food!
I have been teetering about my medicated fish food flake order as buying a larger more expensive amount to cover most illnesses, like all the medications, dewormer varieties and antibiotic flakes as well or just 2 types I might normally need/use 1 dewormer and gram negative medication.