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Our panda cory cat looks really sick. I just checked one everyone and his gills look bloody and red and his fins look bad too. I'll try and post a picture soon if I can find a camera. He's in a 10 gallon tank (sand and minimal gravel substrate) with 3 male guppies, one leopard cory, a snail, and two peppered cories. The temperature is 78F and constant, ammonia 0, nitrate 3, nitrite 0 at the last test a few days ago but I'll test again just to make sure. We change the water every five-seven days. The tank has been set up for 3 months and is completely cycled. Does anyone have any ideas? His gill on the other side looks completely normal and he was fine yesterday.

Looks like some type of bacterial infection.

Try dosing with Melafix. I have seen similar bacterial infections like the ones on your panda that have cleared up using this treatment.
Use the right dose for you aquaria and follow the instructions; but at the same time I would advise increasing the amount of air that flows into your tank when using Melafix. Although the manufacturers say that this is not needed; along with myself, I know a lot of people who do this, as it is our experience that the fish in a Melafix treated aquaria seem to require a higher amount of dissolved oxygen in the water.

Okay, thank you for the advice.

Good advice from Medaka, but a quick question - when you change the water, how much do you change each time?

this can also be caused by excessive amounts of salt added to the water.. Did you happened to add salt when you changed the water?