my tank has been set up since early august. i used the filter material from a cycled tank. the cycle finished and has been stable for over a month. nothing has changed except i removed a fish and the filter sponge into a hospital tank. the sponge was put back on tuesday. last thursday there was 0 ammonia, 0 nitrite and 10-20 nitrate. oh, and i added a bunch of water sprite to the tank on a week ago. today i tested my water before doing the change and now i have 0 ammonia, .25 nitrite, and about 30 nitrate (hard to say for sure, redder than 20, not as red as 40). doing a larger water change than usual.
would having taken the sponge out for a few days have caused this?
would dead plant matter do this? i have had some wisteria die and haven't gotten it all out of the tank yet.
what else would have?