I purchased two sterba's cory cats a couple weeks ago. I have a 30 gal tank with 3 Black Skirt Tetras, 3 minor tetras, and 4 neons.There is also a bumblebee cory that I VERY rarely ever see. The sterba cories do not interact with each other at all. One hangs out at the bottom on one side of the tank, the other is always hovering around the filter at the top of the tank. He will go to the bottom for a little while, then slowly swim back to the top. There are plenty of live plants, driftwood, and rock caves for hiding. The pH is 6.8, ammonia, nitrites, nitrates are all good. I keep the temp between 76 and 78 F. The tank is well established, been up and running for about 2 years. Even the one that hangs out on the bottom does not seem very active. Any advice?