It's a nursery tank 30 cm X 30 cm X 35 cm (high). I've got a fair amount of sand in the bottom, so 25 - 26 cm water height? The recommended amount is 1 drop for every 4.5 litres 2- 4 times per day. I have been using 6 - 7 drops, 2 X per day. I thought that made sense when I was trying to...