Help with Nitrate levels please!

Im having a hard time with my nitrate levels. They go from 20-40 and I have done water changes but they never seem to go below 20.

It is s 10 gal with live rock, corals, 1 shrimp, some snails and hermits and a ball of cheato.

thanks


Here is my log
Date Water Change % Temp °F Specific Gravity
9-Sep-17 79.3° 1.0264
17-Sep-17 79.3° 1.0264
19-Nov-11 82.0° 1.0264
17-Sep-17 82.0° 1.0264
20-Sep-17 80.0° 1.0264
27-Sep-17 82.0° 1.0264
3-Oct-17 50% 82.0° 1.0264
4-Oct-17 50% 80.0° 1.0256
7-Oct-17 100% 80.06° 1.0256
11-Oct-17 0% 80.48° 1.0256

Date Ph NH3 Ammonia NO2 Nitrite NO3 Nitrate
9-Sep-17 8.0 0.00 0.00 5.00
17-Sep-17 8.0 0.00 0.00 5.00
19-Nov-11 8.0 0.20 0.00 0.00
17-Sep-17 8.0 0.25 0.00 40.00
20-Sep-17 8.0 0.00 0.00 20.00
27-Sep-17 7.8 0.00 0.00 80.00
3-Oct-17 7.8 0.00 0.00 40.00
4-Oct-17 10.00
7-Oct-17 8.0 0.00 0.00 5.00
11-Oct-17 7.8 0.00 0.00 40.00