Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Have you tried looking in places that sell rock, gravel, sand and similar materials for masonry or landscape use?
On this side of the pond these are very large stores that sell these materials by the truckload or by the bag. Some of them have graded sand in many sizes and colors.
I have no idea if this is available to you, but look up Pebbletek. This is a material that is added to the finish of swimming pools to make them colorful. If there are any swimming pool contractors (builders) near you, ask them. There may be a similar material with a different name. It is sand-like in size, inert, and quite dense, so it stays down well in the tank. It is available in many colors.
There is a company over here that supplies the masonry yards with graded sand in bags. It is called Lapis Lustre.
I hope some of these names are helpful to you, if not to get some of their materials, perhaps to ask about similar, locally available materials.