Hi Ross,
I think there might have been some confusion with the product names and function. The product you have "Tetra Complete Substrate" which is basically a peat+iron additive that is placed on the bottom and should be covered with any type of gravel you wish. It looks like you've already put the black sand on top which was the correct thing to do.
The product known as "Caribsea Eco Complete" is a totally different product and is a substrate that already has everything it needs without the need for addition of other additives. This product is also black. I'm unsure whether you have both products and had placed the "Eco Complete" on top of the "Tetra Complete Substrate" or whether you just have regular black inert sand on top of the "Tetra Complete Substrate"
You have two choices; 1) You can vacuum off the top black layer and then place the gravel of your choice on top of the remaining "Tetra Complete Substrate" or 2) take both products out of there an just replace with a substrate that is the color and texture you like and that does not require any other additives.
Some popular examples of substrates that do not require additives are as follows:
1. ADA Aquasoil (comes in four colors)
2. Caribse EcoComplete (comes only in black)
3. Seachem Flourite (comes in dark red and light red)
4. Seachem Onyx Sand (come only in black)
5. Seachem Onyx Gravel (come only in gray)
Check this page:
Substrates : Aqua Essentials
You'll see those five products there. All the other products on that page are additives which require gravel on top.
Cheers,
