I have a cory cat that when i look at the tank with the lights off is sitting there just looking dead or turned over belly up. He was always very active and is in a tank with some guppies which i wouldnt think would cause the problem but could help. The tank is a 10g with one cory and like 6 guppies and some java moss in there aswell. Any help would be appreciated

My cories I used to have always did this. Its natural I guess. No problems eating or is less active? if not then nothing to worry about,

Is it only when the lights are off?

WYite

yea only when the lights are out and ive never really seen him eat, just assumed he did it because he has been alive for awhile and is just less active

What kind of cory are we talkin' here and what temp do you keep your guppies at?
I keep corys in all of my tanks with everything from angel breeders to guppies but they are not all tropically inclined. Other than Sterbias the majority of what you'll commonly find at the LFS are cooler water varieties, like mid sixties, not 74-80 F. This is especially true if you want a cory to keep with something like discus at temps over 80, but then you're even pushin' it for the Sterbias.

As for watching them eat, I have some wild caught Julii's that I've had for several years and I've never seen them eat anything at all. On the other hand I have Brochi's and Kronei's that'll practically jump out of the tank at feeding time they get so excited. Overall cory's are pretty durable little critters but when in doubt increase your water changes. Another thing to watch with them is that they require a smooth substrate if you have one. Sharp sand or gravel will eventually wear their barbels (the mustache) off as they root around in it like little hogs. This will not only stress them out but eventually kill them. A nice smooth sand bottom with a few buried black worms and they're in heaven.

The last word on their eating habits is that as with most cats they are primarily lurkers that hunker down in the shade during the day and go out scouring at night. Check your water parameters, make sure that the tank isn't too full for them to get to the surface to gulp some air, drop your temp, offer some food variety and last but not least, get'em some company. They are a gregarious species that do best in groups of at least five or more. They'll exist in hot water all alone, but they will generally not thrive. When properly cared for they often live for many years in captivity.