Adding sand to established sand bed

I have a 29gallon biocube i was wondering if i can add sand to make it a deep sand bed or should i just keep it its about 2 1/2 inch now

 

I would keep it shallow. A DSB can be dangerous to your tank if disturbed.

 

Adding sand to your existing sand bed will kill any copepods, worms etc that are just under the sand bed surface resulting nitrates and hydrogen sulfides that are poisonous for all your critters, areas where water flow will never reach to blow these dead nasties.