I have a common gold fish in a tank with four other mixed fish. The tank is situated in the same room that my tortoise lives in and has consequently an average temp of 27.5oC. The fish have lived in the tank for nearly four years and have never been ill before.
Three months ago the gold fish started loosing its scales and going silver. It showed no ill effects and as it appeared healthy I just left it do it's thing.
About a month ago it went off it's food and started showing signs of a bacterial infection; so I did a course of anti internal bacteria treatment and this reduced the symptoms; it started eating intermittently. I did a 30% water change and another course of treatment, and the fish became more active, but still eating intermittently, even with live foods.
It now had symptoms of a mouth fungus and darkened gills ( the dorsal area and top of its head are still gold as are its gills) I did a treatment for mouth fungus and gill rot and I have been doing this for two weeks now and the fish is now no longer eating, and intermitantly active. On sunday night I moved it down stairs and into an isolation tank and gave it an extremely strong anti internal bacteria treatment containing, formaldehyde which I was recommended to use only in emergencies. It has improved slightly from the move and anti internal bacteria, but is still not eating and only active intermittently. My local aquarist and I agree that by all means it should be dead by now and I haven't destroyed it yet as it is still not lying at the bottom of the tank looking half dead. i had it fourth hand from a neighbour so the age is unknown but all the other fish in my main tank have been completely health all the way through this episode.
Any answers to this puzzle would be gratefully received