Sorry ,had a bit of emergency here at work and had to cut short previous post. As stated i would switch to PRIME water conditioner. It will detoxify Ammonia, chlorine, and chloramines as well as nitrites.You state that the fish do not appear stressed which leads me to suspect that something is affecting the test results considering you record no ammonia but high nitrites and some nitrates. I might also find a fish store that would test a sample of your water preferably a store that uses similar test kit such as API freshwater master kit which is used by many and far more accurate than test strips.There are many reasons that test results can sometimes be less than accurate. Not waiting for the prescribed length of time to compare color charts, waiting too long to compare color charts,not holding the test solution vertical so that drops are uniform, not using enough water and or solution, using expired test kits,etc.
Also, reducing feeding to once every other day or every two days would help and cutting back on amount offered as well.
Layoff cleaning the gravel, don't clean or replace the filter. If filter or sponge gets clogged, swish it around in aquarium water NOT tapwater and stick it back in.
Layoff vaccuming the gravel. IF you aren't overfeeding there is no need to.
Keep the sand stirred or sifted regularly so toxic gases do not accumulate in the sand which can be harmful to fish.
Don't add anything to the water except dechlorinator. No stress this, cycle that, bio,,,ammo,,, etc. many of these products can affect test results and most ,,if not all, are not needed.
I do not know what your exact test results are for they were not posted but,,, If you will adopt some or all of the above things should become easier for you.