high nitrite

I have been trying to resolve a terrible tank crash for several days now.  Nothing I've done seems to help at all.     **What is happening: within 2-3 days after a partial (~10gal) water change my water went from pH 6.2, kH 1, ammonia 0.25, nitrite 0 ... to pH 6.6, kH 1, ammonia 1-2, nitrite...