Best method to add calcium in shrimp tank?

[wllm33]

been trying to maintain just normal fire red shrimps but not much success.

tank is 1 ft and in air con room only and only downoi and seaweed ricchia.
some sand and lda soil and simple water pump and hanging filer.

was told need add calcium. any advice as to best way to add in 1 ft tank and what is recommendation , and freq of wc, once per wk can?

thnks.