They look just about the same to me, if anything the tank tube looks less nitrate. You can see the actual tubes, how to they look to you?
If they are just about the same, that's the lowest you can get nitrate by doing water changes. And the fertiliser isn't affecting things. There are other ways to lower nitrate, but they involve money and/or time.
There are things like Pozzani nitrate filters which could be used to filter the water to remove nitrate before it goes in the tank. This means buying the equipment and replacement filters regularly.
Other people have a vat of water with a lot of fast growing plants in. Water is left in the vat, the plants take up the nitrate as there is no ammonia so they have to use nitrate, then the water is used for water changes and the vat topped up ready for the next water change.. This means having a vat of water standing round somewhere all the time.