blak eye

Hello all,   (Sorry for coming here only when I have a problem...    )   We set up this tank 1.5 years ago, and after the first difficult 6 months it was finally fine, cycled (Nitrite 0, Ammonia 0 - 0.25, Nitrate 20 - 40), peaceful, perfect. Well, almost perfect, since we had only 4 small fishes...