You won't need that much substrate, 3-4" is the maximum; I mean with 4" of substrate how much depth will be left for your fish?! The dark substrate is great for bringing out the best colors in your fish, and it's good that it won't reflect light off causing stress to your fish. If you can't find Eco-Complete locally, go to
Drs.Foster&Smith...they have the best prices around including shipping...