Can't fix nitrite levels in 10 gallon guppy tank?

I have a 10 gallon tank with 2 guppies, one standard and one fancy. The tank is not heated but sits at around 80 degrees or so. When I originally set up the tank, all levels were fine. It has been 3 weeks and for about the last week and a half or so, the nitrite levels have been reading high. I have done several water changes of about 20%, I use aquarium salt, water conditioner, a treatment product which says it removes nitrites, nitrates, ammonia, chlorine, and chloramines and I also added a treatment product used to balance the ph and keep it at a set 7.0. All the products I have used are new, not expired, I do not over feed them- they get one small pinch of fish flakes once a day and I have them in a tank with proper filtration and aeration. I use a 5-in-1 test strip kit and the levels read as follows: Nitrates- Approx 10ppm, Nitrates- 6.0ppm, Hardness- 75ppm, Alkalinity- 140ppm and ph- 6.6-6.8. I have even tested my tap water to see if it was high in nitrites and it isn't. The results come up as next to nothing for nitrites. I don't know what to do. I've tried every suggestion and nothing has helped to change these levels. Am I getting incorrect readings? The water test kit says that it can be used for salt and fresh water tanks. I'm at a loss and I don't want these little guys to go towards that big fish bowl in the sky!!!