As some of you may have seen I've been having some problems with my tank following the completion of my fishless cycle and then adding fish.
To recap, I completed a fishless cycle using the add & wait method, and after 3 weeks the tank was processing 5ppm down to 0 ammonia and 0 nitrite in 10-12 hours.
At this stage I did a large water change (about 80% - yes the water was treated) and added my new fish - pretty much fully stocked, but remembering that all the fish are young and thin bodied, so not up to their full bio-load potential.
Following the big water change I dropped the temperature of the tank back from about 30degC to 25degC ready for the fish (this took about 2 hours I guess).
After acclimatising and adding the fish, I tested the water as a precautionary measure after a few hours, and whilst my ammonia was 0, my nitrite was at 5ppm.
I am currently desperately trying to get the nitrites under control with twice daily water changes of about 38%, but to date the effects of this are limited and too late for some of the fish. Many of the other fish (particularly the neons) are showing signs of distress.
Now the only real reason I can come up with for this is that the reduction in temperature caused a die back of the nitrite processing bacteria, and since they take longer to develop than the ammonia eaters I currently have a dangerously high nitrite spike.
Others may be able to come up with a more plausible reason for the problem, but I would advocate that anybody currently doing a fishless cycle slowly drop the temperature in their tank over a few days (whilst still adding the ammonia) to get to their eventual stocking temp. after they think they have succesfully completed the cycle.
for those who are intersted, the problems are better documented in this thread:
scubadoo's nitrite emergency thread