Although fry damage is a possible scenario, it is really only a theory.
Most research usually predicts fry are most likely to be the victims of high nitrate than adults, anything over 40 ppm can do acute damage, many aquarists accept 20ppm as normal.
In my tanks I try to go no higher than 5 ppm, with the use of every other day 30-40% water changes, and emergent plants like pothos, and papyrus in planted sump/refugiums.
I must, admit your problem has no obvious cause to me.
Most research usually predicts fry are most likely to be the victims of high nitrate than adults, anything over 40 ppm can do acute damage, many aquarists accept 20ppm as normal.
In my tanks I try to go no higher than 5 ppm, with the use of every other day 30-40% water changes, and emergent plants like pothos, and papyrus in planted sump/refugiums.
I must, admit your problem has no obvious cause to me.