Hey all, recently i moved fish out of a 20 gal, and into a bigger tank, and the same day i went out and got new fish for old tank... this tank had been running smoothly for about a year with no issues. After i moved the fish, i took some of the rock out, and redecorated it, added some different plants and a few other things as i re-aqascaped it. To it i added 3 barbs and a bristle nose pleco...right before i added them, i did a large PWC and my numbers were 0 ammo. 0 nitrite and maybe 10 nitrate...about 36 hours later i tested again and i noticed nitrite, perhaps 2-4 ppm of nitrite... so I'm curious how this happened...the tank was well cycled and had a good filter... so what i did is added a chunk of filter foam from another old tank, and then did two 30% WC back to back... i dosed with prime as instructed and the numbers are down now, however there still is 1-2 ppm of nitrite. So I'm curious what could the cause be? Any help would be awesome! the fish don't seem to be showing any signs of being bothered, but i know prolonged time at these conditions could cause major issues, is there anything else i should do? any insight or help would be awesome. thank you!