Avalon,
I have had this problem in my tanks however it seems to come and go in cycles. Certain fish in certain tanks seem to be exhibiting symptoms which are raising red flags in my mind. Sunken bellies, clear poop, eating food but not swallowing it (holding it in mouth for a while and spitting it out), or plain refusing food...
Waterchanges seem to help and probably explain the cyclic nature of this disease in my tanks. I do have medicated food and will probably run it through the gauntlet next week
In the interim I'd like to hear your hypothesis on your treatment, why what how,when etc. . Metrinodazole is something that treats internal parasites yes but does it work through the water column? would the fish not need to injest something. Is the seachem metro special or will any metro work.