what could cause this?

[nemofish2217]

What could cause milli's to slowly die off over a period of one or two months but other acros, monti caps do fine??? I have tried to figure out everything possible! Calc 440, Alk 9, pH 8.3, nitrate 0, nitrite 0, ammonia 0, phosphate 0.

 

[Kenzy]

Could be a combination of lighting or water movement, too little or too much of either or both.