My tank is 178L and is cycled, with 75% sand/25% gravel bed. Ammonia 0, Nitrite 1 mg/l, Nitrate 20mg/l & Ph 7.1. My tank's inhabitants are 6x zebra danios, 6x Golden Barbs, 4x Otocinclus vestitus, 2x armoured shrimps and 2x Dwarf Gouramis'. Its also well planted with wock caves large piece of...