When you say the nitrate level is high...
-increasing the amount of your weekly water change. The amount of the water change should really be based on your nitrate levels
50% PWC will kn0ck 40ppm d0wn t0 20ppm. and s0 0n. S0 if y0ur Trates hit 100 ppm, a 30% WC will 0nly bring it d0wn t0 70 ppm(ish). Certainly spend m0re time 0n vaccing the substrate while pulling 0ut the water.
When in d0ubt read it until it makes sense, then read it again!