Problem with nitrite

I established a ten gallon aquairum from Walmart with a Tetra 5-15
Power Filter.

I never clean the biowheel part and I never leave the filter unplugged
for more
than 1 minute while cleaning the tank.

At first I had gravel but food sifted into it causing the nitrite
levels to show dangerous. I test with strips made by jungle. I took the
gravel out and did big water changes fixing the problem.

Now, the tank bottom is bare and I sweep out food particles daily.
I also have been doing more water changes but despite this the
nitrite levels remain high. The levels in fresh tap water are low,
but nitrate shows a little high.

The other thing is the litle bag that collects debris in the power
sweeper
vaccum thingie is discolored. I tried soaking it in fresh water but it
still
seems discolored. Ive also noticed the water in the tank sometimes
looks
somewhat discolored depending on the angle I look from, but the fresh
tap
water does not.

What is causing my problem? Do I need plants?