assuming your tapwater has zero nitrates, and you are changing water regularly, you need to re-evaluate how much you are feeding your fish.
what goes in must come out, and the end result is nitrates.
rinsing your filter cartridges in tank water is not a bad idea, depending on how long you do it for, but it really won't help your nitrate problem.
if you had an ammonia or nitrIte problem, then that would be something to look at.
but your tank is handling the ammonia and nitrite just fine.
its the end product of the nitrogen cycle that we have to look at.
and to help bring those levels down, its best to increase the frequency of your water changes and cut back on the feeding.