Possible but I wouldn't lean in that direction. The damaging effects of cyanide are usually internal but can still definately affect the gills. The reason I doubt it is the fact the fish recovers. If there was enough cyanide to affect the gill in this manner, the fish would be dead. The "saturation" if you will of the gills would be proportionate to the brain and the liver. Both the brain and the liver would be the worst affected so while it may be possible, it does not follow true with what we know about how cyanide makes it's way through the fish's anatomy.
The "theory" if you will that has been running through my brain is that the fish is introduced with some sort of worm (digenean) or through the addition of another animal, has introduced it. Given the often complex nature of the worms reproductive life, the problem is abating itself due to the lack of necessary intermediate hosts.
