How much sand

[PineHill]

This is the first time I will be using sand in a tank rather than gravel.  I always like to have deep gravel but since sand isn't as porous I am worried about bacteria growing deep in the sand.
Is there a specific range of depth that I should put in my 55G tank?  I will have eggcrate under the sand as well.  The tank will be housing frontosa and lemon chiclids.
Thanks

[sas]

This is what we ended up doing, keep in mind that I've only started in with Africans since last fall so no experts for sure.
The eggcrate went down first, then the piles of rock and then the sand went in. The rocks sit directly on the eggcrate so no chance of shifting. I only used enough sand to hide the eggcrate, about 1/2 an inch maybe, for now since I need to get some snails to turn it over. Once we get some lovely MTS I'll add more sand, another 1/2 inch or so?
Hope that helps .