I normally use two. See how it goes. If that seems okay, I'll continue with that amount every six weeks to two months.
75g freshwater - Fluval FX4
Fluval Plant 3.0
1 Black Marble Angelfish, 7 White Skirted Tetras, 7 Black Skirted Tetras, 9 False Julli Cory, 3 Pepper Cory, 8 Green Cory