"Victor M. Martinez" wrote in
:

Yesterday one of my mature SAE died. Just now I fished one out of the
tank. The rest are hiding in the large amazon sword. I did notice they
were "breathing" really fast lately, but I didn't think any of it
because all the other fish were acting normally.

They are suffocating. When the oxygen level is down, SAEs are first to die
because they need more oxygen than normal fish, they come from fast flowing
rivers. Quickly add surface movement by adding a powerhead or placing the
one you have so that the water surface is really moving. The arranging of
plants and digging bottom might have something to do with oxygen imbalance
in your tank.

Liisa