Don't really understand my results/sequence of events.
Have established nano tank 35L. Upgrading to 120L tank currently right next to it.
Added large second filter to nano tank, primed with bactinettes and ran it for 3 weeks.
Set up new tank, added a bit of gravel, 2 plants, a decoration and second filter from old tank. No rush but expected to be able to transfer fish pretty quickly.
The next morning the tank was cloudy, and in the evening was so cloudy you couldn't see the back. Fine I thought a bacterial bloom which will settle, and tested. Ammonia 0, nitrite 3.3 and nitrate 110. I now realise the nitrite was not as high as peak in true cycling - but having been used to very stable parameters in old tank for some time (NH3 0, NO2 0, NO3 10-20) to me they were "off the scale". The really puzzling thing is that I hadn't added fish or ammonia!
Where did the nitrite come from? I guessed there must have been enough biological material with the new substrate and plants to create the nitrite/nitrate I was seeing. I thought the levels were high so I didn't add ammonia and watched the tank for several days expecting the nitrite to drop as the bacterial population increased. But it stayed exactly the same for five days. I did a 50% water change to "kick start" it - both reduced but didn't go to zero (nitrite down to 0.6 to 1). NH3 was zero throughout.
So at day 7 I concluded that the filter hadn't transferred enough bacteria, and by now they were likely gone - so added ammonia. Used a calculator and correctly got the level up to 3. 12 hours later it was down to 1. Added ammonia up to 4 and then took 24 hours to get down to 2. Nitrite is now 1 and nitrate 10.
So - all rather puzzling. Why didn't the old filter kick in straight away to reduce the nitrite? Where did all the original ammonia come from since I hadn't added any. Whatever - I think I am now in a cycling process, and will continue the add and wait. Wonder why ammonia dropping more slowly on second addition and why I haven't got the ammonia completely to zero yet, and the nitrite doesn't seem to have increased as much as it did initially (although first few days).
Cloudiness cleared within 48 hours, and tap water doesn't contain nitrite. Tap water has some nitrate but not high (around 10). pH has been stable throughout around 8.0 (hard London tap water), and no filter charcoal.