Popeye (not the sailor-man, toot toot) 
This is just some information I wanted to share with Bob Fenner. I've been reading the faq's for a long time, mainly the ones on Popeye and wanted his input on something that was happening in my tanks quite a while ago. It seemed that every new fish I'd add to my tank would bring on a case of scratching and shuddering on a few of the inhabitants already in the tank and sometimes the new addition as well. There would be no obvious spots on the fish other than occasionally a cloudy eye. After killing a cream angel through stressing it with all sorts of treatments I decided to try freshwater dips on all new victims. What actually fell off the fish were flat transparent oval parasites like mini lenses ranging from 1.5mm to 4mm and totally invisible in the main aquarium but would immediately turn white in freshwater and fall to the bottom. They seemed to be mainly on the fishes eyes and the eye would only turn cloudy and swell plus possibly develop a bubble if there were 2 or 3 on it. I have just purchased a blue ring angel over the internet and had it delivered even though the vendor informed me about his Popeye, after leaving him in a tank on his own for over a week to recuperate I decided to give him a freshwater dip and guess what, three of these little contact lenses fell off his eye and the Popeye is gone. My question really is why is this such a common problem with me but in all the internet Popeye theories I've read I've never heard anyone mention these mainly eye parasites as a cause of Popeye with an easy treatment. 
<Mmm, don't know, or am not so sure this is such a common link... cause-symptomatically> 
for months in my main aquarium these little things would return and the fish would scratch and shudder occasionally and a little flat disc would just about be visible on the eye of one or more fishes, this happened on a monthly cycle and has now stopped altogether. Your thoughts on this would be greatly appreciated as my local shops don't seem to know what they are even though I have taken the parasites that fell off the fishes with me in a small jar. 
best regards Archie 
<Thank you for sending your notes, observations along. I do suspect that the disks you saw were trematodes (flukes)... and that they might be linked to the symptoms you note. I am a BIG FAN of pH-adjusted freshwater dips for most all marine fish moves... as my long writing history will testify. Will archive your input for others edification. Bob Fenner>