[somewhatshocked]

Re: adding sand on top of existing substrate...

Just know that if sand is heavier than the substrate you've already got (I.E., Aquasoil, Fluval Shrimp Stratum, et al), it'll end up beneath your current substrate bed in no time flat. Same for substrates that are larger in grain than sand and allow for it to fall through the cracks (things like Eco-Complete, pea gravel, Akadama).

 

[somewhatshocked]

Not long. The sand will definitely settle beneath Eco-Complete.

 

[somewhatshocked]

Whether or not gravel mixes with Eco-Complete likely depends on the weight of the gravel, its size and such.

Any particular reason you're trying to cover up the E-C?