New to the community. Been keeping fish for about 20 years. Recently got some Altums. In a week I lost all eight of them. They all hung up top of the tank most of the time. They ate well, but one by one they quit eating and died a day later. It is a plant tank. Temp is at 86. Initial pH of about 6.8 was dropped to 6.4 after adjusting with acid buffers and daily water changes. The tank is a 60 gal cube. I'm trying to figure out what could of happened. Any advice?

