Sad morning. Last night I noticed one of 11 tiger barbs turning sideways and swimming crazily. He seemed like he lost color and was “shiny”. After dinner proceeded to do a 50-75% water change and saw more tigers following the same symptoms. After the change, 3 had died within that time. By the end of the night 5 total had died. 6 left. I should side note I had 6 originally since the tank cycled and added 5 about two weeks ago, and the first ones to die were all the new ones. Now the 6 originals were left. By morning all were dead. All other fish in the tank have been ok with no signs of death. What is my problem here?