I hope the sand you are adding isnt live. That might cause a spike in ammonia. Did you try turning off all the flow then slowly add the sand using a small container or something?
use a glass you have of course rinsed out well. pack the sand in it loosly, then put your hand over the top and guide it down to the bottom and tip it over slowly. Works great. I did it in my 35g and didnt have a cloud at all.
When I added the sand to my tank, I bought a wide mouth funnel, then stuck it on a piece of 1 1/2" PVC so I had a funnel with a long pur spout. It let me add the sand where I wanted it and did not create to bad of a sand storm.