You might be better be better off using an aquarium plant substrate instead (such as Florrite or Eco-Complete). Soil-based tanks are a subspeciality and unless you are very competent aquatic gardener already, you could turn your tank into a toxic cesspool.
Tiger barbs are highly active schoolers and moderately fast swimmers, so maximize length of the tank over height. They are tremendously adaptable and hardy (especially if you stick to the wild type color form), so substrate is your choice. The more fish in the school, the better the group will be. To me 8-9 are the absolute minimum, and twice that is twice is good, so we are not talking about a small tank. The front half of the tanK should be kept as clear as possible for maximum swimming distance, the back corners and all across the back can be planted as densely as you wish. The colors will show up best over natural gravel or Flourite IME. Light bright gravel will wash the fish out a bit.
HTH