Some background....
60 gal
10 Congo tetras, 4 kribs,1 A. Thomasi, 1 African butterfly fish, 1 African brown knife, 5 another smaller S. American tetra forget the name, 1 rubberlip pleco, 1 Colombian pleco, ~ 5 assassin snails, 2 upside down cats
6.8 pH
0 ammonia
0 nitrite
0 nitrates...