Earlier today I did a 25% water change along with vacuuming my gravel in my 55 gal. Everything went smooth I let it sit for awhile and then I tested the water since I got those 5 in 1 test strips. Placed it in there and where I am from the water is really hard so I have high GH and KH but my NO2 was at 10ppm and my NO3 was at 200 ppm. The current stock in that tank is a lmb channel cat a common shiner and then two pumpkinseeds and two green sunfish. The largest fish in there is 4.5" so its not over crowded. While sifting through the gravel I picked up some pieces of food and crap but I can guarantee I got 98-99% of it. The tank is also planted. So I went and tested my 20 gal which has two 3" cats in it that are growing out. Now this tank was a little messier from feeding them frozen bloodworms and live minnows but my nitrites and nitrates were 0 ppm. I dont get it I feel like the strips are wrong since my fish look fine. Can anyone give me a hand here?