We have a 50 gallon native tank with some small brim (4). One by one they turned black on the top half and their mouths were locked open. All 4 died over a period of 3 days. When they died they didn't float to the top. They sunk to the bottom and were all belly up. I know people will need more details to answer this so I'll wait to see what needs answering before I add anything else. PLEASE HELP!!!