I have a 55 gallon tank with Angelfish, Cardinal Tetra, Cory Cats and an Emperor Pleco. The tank has been up and running for almost two years now, the water tests fine and I do a 50% water change weekly. I have done a couple of different stocking schemes with it but I am very happy with my current setup. The tank has smooth gravel, lots of driftwood and is fairly heavily planted. I have never had issues with fish dying and am stumped by this one. I have had 6 leopard cory cats for about a month now. Over the past few weeks I have slowly been picking them out of tank, and today I removed the last fallen soldier. They were very active for the first week or so, as expected, and then they just started laying around doing nothing. I make sure food reaches the bottom for them and they seem to be eating. What is totally blowing my mind is the Angelfish and Cardinal Tetra are completely fine and I would think they would have health problems long before the cories, but I have not lost any of them. I need to figure this out because the cories complete the biotype and I really like the activity of healthy cory cats. Also, my plan now is to do Sterbai and they are running about $10 a fish in my area.