So last night the 4 month old fry took a really bad turn for the worst. I decided to
euthanize him last night. He started looking better after I got him into the nurse tank but over the past few days reverted and then got worse. She started swimming in very sporadic and enthusiastic bursts but looked more like a coin flipping through the water than a fish. I saw he was trying to eat but couldn't so I made the decision to end his troubles.
I froze him last night to ensure his fate and looked him over a minute ago. He appears to have some kind of development issues. His back was raised into a hump and his face looked squashed up like a bulldog. When I removed him last night I noticed his head was about twice as wide as the other molly in the tank and he had a rosy coloration between his eyes which is weird for silver mollies. I can only conclude that his issues were not caused by disease but by some genetic abnormality that likely caused some form of retardation. I suppose this happens even to fish. None of my other fish are showing any signs at all of illness and I have several in the tank the same age and some younger.
I am not upset because I feel I did him a service of taking him from this world peacefully as opposed to being harassed to death or starving. This was quite a learning lesson and I will take more time to examine sick looking fish sooner as to avoid this kind of thing in the future.