I need Help

[RomaFan]

Hi everyone, I could for sure use some help.I have a biocube 14 that has been up and running for over a year now, its a berlin setup with sps and two percs. The Sand bed is about 3 inches deep, with 25lbs of live rock. I have a lfs back pack skimmer and radon lighting.
Parameters a re as follows:

SG35 ppt
Temp 73F
Cal 500 ppm
Mg 1500 ppm
Alk 10.2 dKH
Nitrites 0
Nitrates >60ppm
Ammonia 0 ppm
Po4 0.03 ppm

I have been battiling very high nitrates for about a month now, I have done every other day water changes, I have been using red sea NoPOX, to no avail. I have looked for dead debris and not standing water, there is very good flow. I am at a lost with this. I have been thinking if this could cause my problem:

I have a pistol shrimp and he is moving sand everywhere every night. Keeps digging and making tunnels, and with a small footprint can this cause the high nitrates? Needless to say my sps are not very happy and neither am I. Any suggestions would be appreciated.