First, clean your canister.
Try taking the rock out and setting it in buckets after shaking them off in the water.
Then, swirl the water around in the tank and get as much detritus suspended as you can and do a large water change.
Replace the rock and let the canister remove the rest of the detritus from the water column.
Clean the canister again, and retest.

But, before you do all of that, take your water to be tested at the LFS.