Originally Posted by Raul-7
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...
He's setting up a 90 gallon tank so he should have plenty of room left for the plants and fish. Lots of people (think Amano) aquascape their tank with very radical changes in substrate depth, it gives an undulating (rolling hills) feel to the 'scape. Disclaimer: this is coming from a guy who has substrates that are less than 3 inches deep.
The only thing I would caution against is using a lot of shallow rooting plants over the deepest substrate. You want you deep rooters there so that the plants do what they do best and keep the substrate oxygenated. While I can demonstrate that plants can overcome an anoxic substrate, one that has decomposed to the point of being toxic is dangerous to your plants.
This is coming from an aquatic ecologist.