dieing fesh water fish i need help!! new to this

[mikeurbeck06]

i have a 120gal tall i have angelfish plecos rainbows tetras and some catfish ill have 1 or 2 fish die off every few days for my 120 ive had the water tested and every fish store says its great water conditions and the water temp is always around 78 or 80 degrees they seem to last a wile (few months) befor they start to die off the 1 large angelfish i have is like my best friend were always greeting each other and hes one of the few fish thats lasted the longest about a year now his bodys about 3 1/2" around then add his fins so hes about 6" tall tip to tip any ideas on why there dieing off?? i buy from nice clean fish stores that hand select there fish also..