I have a 46g bowfront that I just switched over from having 2 adult yellow labs, a Alt. compressicep, an unknown adult pseudotropheus, 2 clown loaches and a bushynose Pleco to having 4 Aust. rainbows, same pleco, same loaches, 2 cory's from my daughters 10g which we took down. I reduced the bio-load by doing this. It has a fluval 404 and all I did was clean the filter during this switch. Which it was about 2 weeks over due. I rinsed the media & sponges, refilled & returned to tank. No water change.....nothing. I added 5 plants (to see if my well water is okay for plants) and removed about one third of the rocks so it would be a little more open. (all of this yesterday) Today I test the water and my amonia is at 0, Nitrite 0.3, Nitrate 25. (usually it is around 15-20) I planned to do a water change today anyway as it is a week overdue. (started a new job and it's been hectic). I was surprised that I have a nitrite spike. All fish are accounted for , when moving rocks I didn't notice any buildup of gunk. I did disturb the sand more than usual while putting plants in, could this have caused a problem?