Best thing to cover coco to protect roots?

[ThatRapperWeed]

A couple of my plants have slopes on the top of the coco causing coco and perlite to run down the slope sometimes exposing roots when I water. Is it a better idea to top with a bunch of coco or a bunch of perlite?

 

[TripsRabbit]

Sand helps control pests, it smothers fungus gnats.