Lighteners in order of preference for indoor use:
Perlite- the best! The white crackly stiff in potting mix
Vermiculite- Also good- looks like mica- sorta
Sand- Not the best, but you could use it in an emergency. I don't recommend it though.

Lighteners improve drainage and aeration of the soil. You REALLY need them indoors. Pre-mix them before putting the soil in the pot.