i used a 25kg bag of gravel to cover the floor of a jewel 180 aquarium and that came to about an inch of cover all over which is adequate to plant the plants. probably in hindside a 30kg bag would of given me more depth at the back but iam happy with the result. 10-15kg would be ok. if you get enough to start of with about 1.5 inches to 2.0 inches at the back going to 1 inches at the front.