Hello friends:
Last winter I only used one pressure filter (nominally for a pond with a volume of 6 m3 with koi) to my 1-1.5 m3 over-wintering tank (volume varied due to slow water addition), and the water turn-over was 6.5 m3 per hour. I got nitrite reading all through that winter, 0.2-0.8 ppm.
This winter I have more than doubled the bio-filtration by adding a flow-through filter nominally for a pond with a volume of 9 m3 with koi behind the pressure filter, in tandem. I also increased the water turn-over to 12 m3 per hour. The system has been up and running for about 3 weeks, but I still get nitrite reading, 0.2-0-8 ppm.
Every other condition has been similar: Roughly same amount of fish, same air pump, similar temperature (17-20 centigrade), similar amount of NaHCO3, and the amount of feeding is also similar. So I wonder why I still get nitrite. May it be caused by the notoriously long time it is needed to pass the nitrite oxidation step, as the flow-through filter had been used through-out the summer but had suffered from cold temperature (about 5 centigrade) before being moved inside.
Thanks for your thoughts!
(I know that the ingenious sand-and-gavel pre-filter, rotating drum filter and the legendary shower filter would do the trick, but I donīt have these right now).
A picture of the set-up: