Hey Lifeoffroad,
Terrance was a little too straight forward when stating that you overfeed, but I do somewhat agree having kept reef tanks that require pristine water.
I don't see it as overfeeding so much as just feeding more often. The fish like it
Some people enjoy feeding their fish more regularly which is usually* just fine as long as they don't over pollute the tank while doing so.
The truth is many fish can go for up to 2 weeks possibly longer without being fed. I have seen more fish die from feeding too much than not enough.
I also agree that there is a lot of organic waste and oils or "soup" in frozen foods such as brineshrimp, bloodworms, mixed foods..ect. The amount of "soup" is usually less with higher quality foods. I have always rinsed my frozen food with a small fish net then added it to the tank.
If you are truly having a nitrate problem in your tank (its not the test kit) and have nitrates in your tap as well... then I would recommend reduce the amount and/or frequency of feeding. If you are totally opposed to that then just keep doing large water changes. You could also buy bottled water or distilled water from the store and mix it with your tap.
Also have you considered live plants?
Anyways good luck! I'm in no way trying to tell you what you are doing is wrong, just sharing my personal experience