Question

what should you add to your ecolumn to create the decomposition layer?
mosquito fish
water and soil
dead or decaying leaves and plant matter
only sand

Explanation:

Decomposition layer in an ecocolumn requires organic material for decomposers to break down. Dead or decaying leaves and plant matter provide this organic - material source. Mosquito fish are consumers, water and soil alone don't provide enough organic material for decomposition, and only sand lacks organic matter for decomposition.

Answer:

dead or decaying leaves and plant matter