You haven't mentioned water changes yet. Just adding water to replace evaporation will lead to a buildup of nitrates over time. Not a problem in low amounts (unlike ammonia which is deadly) but in high amounts is toxic. If you aren't doing water changes, then I'm going to venture a guess that your Nitrate reading is off the scale.
That kit's cheap at any fish store.
I did drain it almost entirely in October, I think and then refilled it. I'll do a 70% or so swap just to be safe.