Advice Needed

Hi fellow bros,

I am starting my own 422 planted soon (but after more read ups first), had a few questions in mind, request the assistance to clear my doubts

I had some scaping in mind, but require some glue and sticking. approach a lfs and was told to use water friendly silicone. Came across this in c328.

1) will the silicone be safe for the fishes and plants? Will it emit any harmful or poisonous substances?

2) Can styrofoam planks be used to create the depth and volume of the tank rather than filling up all with soil?

3) Read earlier that can put lava rocks in between the layers of gravels and substrate(cant locate the post now). What benefits does this have and how will it affect the water?

Thanks in advance