Why can't I keep fish alive...

[pinhead]

I would agree with a previous post about checking your oxygen levels. My tank is established and has been running for a number of years. All of a sudden one morning I had a dead pygmy angel and a tang in distress. The tang recovered later in the morning after the lights had been on a while. I had repositioned a power head a few days earlier and the circulation at night had changed with not as much flow at the surface.

As the corals have grown larger, they are now requiring more oxygen at night for respiration leaving less for the fish. By altering the flow there was now too little for the fish. I think Angels are particularly sensitve to oxygen levels. The only way I was able to figure this out was seeing the Tang recover. If they hide in the rockwork, you will never see them in distress and may never find their bodies.

Adjusting the powerhead to ensure surface agitation at night solved the problem.