I've been keeping fish for coming up to a year but I've never had to run a cycle as my first tank was second hand and already matured.
However, I recently bought my second tank (Fluval Edge 46l) and thought I'd chuck the bio media in the rear sump of my main tank (AquaNano 40). For more context, I've also god around 500g of Fluval Bio Rings in the rear sump in addition to the stock media.
I left the new media in the established tank for around a week and last Saturday transferred it back to the Edge along with some of the established bio rings. I also squeezed some of the sponges into the Fluval to get as much biofilm as possible into the system.
After letting the tank clear, I then added my three juvenile mollies (almost 2 months old) into the Edge and they loved the space. However, I tested the water the following morning and found high nitrite readings of what looks like 2ppm - 5ppm (it's hard to tell with the API liquid kit).
I immediately did a 25% water change to being the nitrites down and returned the mollies to the main tank just in case. The nitrites dropped slightly but they're now at the lower end of the 2ppm - 5ppm spectrum. For the record, ammonia is reading 0 or trace.
My question now is what do I do? My tap water naturally has a high nitrate content so I can't monitor whether the nitrite is converting other than checking if it's gone down. Do I do more water changes? Let it lie? Feed the empty tank for more ammonia?
Appreciate your thoughts. Picture of the tank for reference!