Water Chemistry Guru's

[yargo]

I have a water chemistry problem that is baffling me. My nitrates are high @ 20ppm. All the other water parameters seem to be OK. The tank has been running for 11 months. I’m using fluidized Bio-Pellets; they were introduced to the system about a month ago @ the minimum recommended dosage. Water changes are every 2 weeks at 15% minimum. All tank inhabitants are accounted for so the death of something isn’t causing the spike. Does anybody have any suggestions on what would be causing the spike & how to correct it?

NH3/4 (ammonia) 0.0 ppm
NO3 (nitrate) 20 ppm
NO2 (nitrite) 0.02 ppm
PO4 (phosphate) 0.01ppm
PH 8.0
DKH 7 (a little on the low side)
Ca 400 ppm
Mg 1300 ppm