I have always in the past refrained from attempting to make any definitive diagnosis of fish health issues. This has been because: 1) I am not a veterinarian, 2) Many fish diseases exhibit the same symptoms, 3) It is all but impossible to make any legitimate determination without being on site and observing the infected fish and its immediate environs first hand.
That being the case, i will still offer 'what-ifs' such as the Ammonia spiking after feeding. Which is a fact-
"Since ammonia is a byproduct of the catabolism of protein, it is strongly influenced by feeding and subsequent metabolism of the feed by fish and bacteria. Ammonia typically peaks about 90 minutes after feeding."
http://web.utk.edu/~rstrange/wfs556/html-content/05-ammonia.htm
After re-reading the reliable resources on fish disease, it appears that Costia may possibly be the cause of Jenny's problems. I would suggest that this be investigated further and if satisfied that this may be the problem, then treat as recommended. Check this site first-
http://www.fishdoc.co.uk/disease/costia.htm