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I have a few questions. Any help would be appreciated. I have a 36 gal. freshwater tropical corner unit tank that has been establlished for about 2 years. I think I have too much gravel on the bottom of the tank. I only have a top bio filter. I think I misread the instructions when I first set up the tank. I put the amount of gravel the book suggested for an undergravel filter. I have about 2-3 inches of gravel. I have a community tank and will go for long periods of time without losing any fish, then I start losing about 1-2 every other week. I perform regular monthly gravel vaccuumming and weekly water change. Can I remove the gravel that is in there and leave about a 1/4 inch without tearing down the whole tank? I it possible that even though the tests come out normal at pet stores and the water is clear that I am unable to really get the gravel clean? Also, I have had 3 black spotted cories since I have had the tank. Just today I noticed that one of them has a red "hole" behind one of its gills. It has been swimming all over the tank, which is completely out of character for it. I read one website that referred to this as "red spot" disease specific to cories and it said it is highly contagious to the other cories, but not any of the other fish. The article referred to the "plate" of the catfish. Is a plate close to the gill or is it another name for the gill?