I'm currently battling a weird problem in my 55 gallon, nitrites. Tank has been running for three months, First three weeks fishless, had the Bacterial Bloom. Started the beginning of the second week in September. On the 2nd of October I added two corys and a pleco. On the 2nd of November I added three more Corys and four Angelfish. I was doing a 25% water-change each week until the week after I added the AngelFish, then I stepped up the water-changes. The last five water-changes I've done were 50% water-changes, two in the last four days.
I'm using a fresh API master test kit(Expiration date is 2017), every time I get the same readings, 0 ppm Ammonia, 5 ppm nitrates and a whopping 5+ ppm of Nitrites! I've been running a pack of Nitra-Zorb in the filter for the past couple of days and treated with Quick Start(Shows how desperate I'm getting) today. I'm going to test again tomorrow and if it's the same, another 50% water-Change.

Have any of you run into this before?