Sick Cory? Help!

[FlakeyBiscuit3.14]

Okay, so I got home from work tonight and one of my corys was sitting up against the glass, fins down, and now has not moved for hours. The other corys in the tank are all doing fine and are acting quite happy and healthy.
 
29G tank, with bloodfin tetras, platies, ghost shrimp, a snail, and the corys. I have 5 corys total. The tank I've had cycled for at least two months, and the corys for at least a month now. The now sick cory is one of the smallest of the bunch, with two of the 5 (all bought at the same time and same age) being about triple the size of this poor little guy. I know I have witnessed all 5 corys eating before, though yes, some more than others. I feed tropical flake and hikari bottom feeder pellets daily, and freeze dried bloodworms about once a week. Since adding the platies (a few weeks ago), I have been doing either one 50% water change a week, or two 30% water changes a week. Most recent water change was yesterday. With a liquid test kit, I consistently get readings of 0ppm ammonia and nitrite, with nitrate being usually 10ppm before the water change. The tank has very fine sand and live plants and driftwood. It is kept about 75-76F. The cory may have slightly red gills? but no missing barbels. All other fish acting normally with no other signs of sickness.
 
I have one other tank that I could move the cory to, but it is a 10G tank with guppies and fairly coarse gravel substrate. Not sure if I should risk moving him....
 
Any ideas??? I feel so bad for the little guy!
 
 

 

[FlakeyBiscuit3.14]

Forgot to mention... my pH is 8.3, the only "chemical" I add to the tank is root tabs, but last ones were added a week ago, and no new additions (live or decoration) have been added since the platies over two weeks ago. Really, the most recent thing is the water change I did yesterday. I did do a 50%-er, and moved around some of the decor to get all the platy poop that hides everywhere, so I know that was stressful....but if that was over 24 hours ago could that still possibly be affecting him tonight?

 

[mark4785]

I would move the sick Corydoras to the 10g aquarium as something, in my opinion, has had a negative impact on its health during the tank maintenance that you carried out. I once re-arranged some ornaments in my aquarium and cleaned the sand by syphoning it and this seemed to cause some of my corydoras to develop the pop-eye bacterial infection. I think cory's are treasured for keeping the gravel extremely clean because they gulp it up and eat any decaying food and anything they do miss comes out of their gills and usually floats into a filtration unit. Thus, if you dirty their environment by releasing dirt from the gravel as you move it and any ornaments around I think they can become exposed to diseases and pockets of ammonia that would otherwise have not come into contact with them.
 
Your other cory's should be fine but the one that is ill needs a very clean environment while it recovers and you need to look out for loss of appetite, pop-eye, pinecone gills, fin/mouth rot and redness, all of which may occur if a fish has been privy to some toxins in the water. As you are saying it as red looking gills, you may want to apply an anti-bacterial treatment fairly soon to fight the infection.
 
Edit: I would try to lower, very slowly, your pH level to something like 7.5 as the pH of 8.3 may be irritating their skin. You can lower pH by adding catappa leaves or peat.