I took the clay pot I had in my tank out on Monday, and today, I had no nitrites. So, I guess that the pot was affecting the cycle. I miht get a different one and try it.
if i'm on the right track i think NJ said the pot hadn't been used in a previous thread. if i've go the wrong end of the stick here then just ignore me
anyway, if the pot is responsible then it must have been used or at least come into contact with something nasty. the only other explanation i can think of is that the addition/removal of the pot is a pure coincidence happening at the same time as the start/end of a nitrite spike. this seems unlikely.