Recently set up a tank.38gal. My first go at a planted. It is currently low light until I get my medium light. Not a large amount of plants yet either. Anubias, water sprite, some grass and a sword(doing great despite the low light). When I was cycling the tank I gave it a jump start with a huge squeeze of a filter pad from my established tank. I also put in a used filter from another tank with the current filter. It converted ammonia very quickly. Tested daily - all good - added some fish, not a bunch; 13 neon tetras.
Tested today and ammonia is at 4. I use the api liquid test. Super green color. Tested other tanks to double check myself - good. Did a 50% change. I use prime. Fish seemed very normal. What gives? What can cause a spike like that?
I had an ammonia spike in a different tank before but that was from insufficient bacteria. I can't imagine 13 little tetras loaded up the tank. Any thoughts??
Thanks