OK today was doing my standard weekly test to see if my nitrates had dropped below 20, Ammonia 0, then went to nitrites, as soon as the drops were done, BAM purple. So i started to panic, test my refill water source, 0 Nitrites, Did a 70% water change and my reading are still showing off the chart. LFS tested and same reading. What would've done this. Nothing has been added in the tank for 2-3 weeks except a 1 inch mandarin. Any ideas? Doing another change tomorrow to see if i can fix it.