Look for an Octopus. I assume you do not have a sump.
However a PS is not going to cut down on the nitrates all that much. Your cannister filter is trapping detritus and breaking it down to nitrate. That's its job. Add double the amount of LR
to get to the 1˝ pounds per gallon of tank capacity and use that as your bilogical filtration. Then you can stop using the filters inside the cannister. That will help.
Double your PWC
's to be 20% per week. Nothing works better to cut down on nitrates and clear up water quality issues like a 70% or better water change over a month.
Next would be cutting back on your feeding. How often are you feeding?
Cut that in half too!