son of a...

you get the idea.

i got a trio of zygatus cories a couple of weeks ago. we quarantined for a week and did not notice anything unusual and so put them in our 55 community. all this week theyve been active and eating. and then this morning my bf noticed one of the zygatus was sitting all kinds of weird; he was rigid and sitting perfectly still. we netted him and quarantined him and he died within a half hour. now weve noticed another of the zygatus is behaving a little lethargically and is surfacing to gulp air more often than typical. the wierdest thing is both the cory that died and the oddly behaving cory both seem to have pale eyes. not cloudy like in cloudy eye, popeye or an injury; the iris has turned white, but the lens over the eye is clear and the pupil is clear and almost appears dilated. they dont have any funny growths, they arent bloated. the tank was not being treated for anything. the temp is at 78. the pH is 7.2, ammonia 0ppm, nitrite 0ppm, and nitrate 20ppm. we did a water change after we noticed the first sick cory. we use prime as a dechlorinator. the tank has been up and running with fish continuously since april. aside from the two zygatus, everyone else seems healthy and happy. any ideas what this could be?

here is the healthy zygatus (behind the peppered cories to the right). his iris is dark like a typical cory's:


and here is the diseased(?) cory. i know its hard to tell, but his iris is all pale and it looks like hes staring:

the parameters are the same. i checked. pH is the same, no ammonia or nitrites, nitrate at 20ppm. temp is at 78. i dunno. ive never had any that did this. i got a trio of tiny similis from the same lfs and they are doing fine. in fact, we noticed one of the similis had disappeared for a couple of days and ripped the tank appart to find him. turns out, he got sucked into our overflow and down into our sump where he spent 3-4 days among the plastic pot scubbbers. hes a little ragged, but is fine. i dunno

also, the eye color is still screwy on the one zygatus, but hes still up and breathing this morning