Sick cory cats?

Question
I have a 10 gallon with two cory cats ( I am not sure which type). I have had them for 4 weeks. both of my guys are sick and has been for about 2 days. they lay on their side on the bottom and don't seem to have the energy to move.When I first saw them that way I assumed that they were dead but up close noticed that they still breathing. They have had endless energy since about 2 days ago when this started to happen. They are in a 10 gallon tank with 2 black tetras, 1 pleco and 2 septae tetras. The tank has been going for almost 2 years now and I change the water 20% faithfully once a week and clean the gravel at the same time. I check the Ph every 2 days and adjust accordingly. I have no idea what is wrong with them as I'm new to fish. Any ideas as to what could be wrong?  

Answer
If they're laying on their side they should be having problems digesting some food. Specially with the sudden onset. Boil some peas and pop them out of their shell and feed that to your fish. It wont effect any of your fish to live on peas for a couple days. See if that fixes the problem. Peas help fish pass indigested food :)

As far as your water changes, when you say you clean your gravel, I want to warn you; Things grow on your gravel that eat the ammonia in your water. Its important that if you use gravel that you dont effect it to much, just get the LARGE items off of it.

Good luck with it
Andy