What kind of substrate?

[Cheesetian]

I've recently bought a 2x1x1 and i think it's about 15g. Bought it for the sole purpose of trying out a planted tank with a couple of dwarf snakeheads. Was thinking of carpetting the entire thing. What kind of substrate would be good? Specifics would be good and instructions on how to do it would be much appreciated as well, things like which goes in first, substrate>water>plants or whatever the correct steps are. And would the plants irritate the snakeheads? Considering they're almost as good as bottom dwellers, especially the smaller ones.

Thanks in advance!