It doesn't need to be capped... lol
It's a nutrient rich substrate in itself. It's really nice to scape with, just spray it with water and it stays pretty well.
I don't really know your tanks translation into gallons, but I'd get a 9 liter bag (better value) unless you want to scape a very small nano (~3 gallons). A 9 liter bag let me scape a Mini-S with a 5 inch slope in the back and 2 inch in the front, and a 3 inch deep 10 gallon tank.