I had a similar issue with a school of tanicts in my 6 month 20 gallon tank, and that seemed to be excess of parasites, bacteria and fungi.

My fish had three symptoms. Some of them had hollow bellies even though they were eating daily, some had their tails rotting, and some frequently rubbed themselves against stones and plants (a sign of ich). The ones that had hollow bellies died.

I think that the root cause of this was poor maintanance of my substrate. I had rather not siphon glossostigma judging that this would harm the plant, but that seemed to be a mistake.

On the front corners of the tank, a lot of mulm could be noticed within the substrate.

I performed 60% water change thoroughly siphoning glosso by pressing the siphon on the top of it, throughout the substrate. This procedure removed a lot of mulm and hundreds of a tiny crustacean which species I don't know.

Also let UV turned on for 4 days.

In 2 weeks, fishes are not flashing on stones any longer and their tails are recovering.

Now, siphoning glosso is part of my maintanance routine.

Regards,
Evandro.