Originally Posted by Despresamy
I have no clue what the nitrogen cycle is
Ok I will explain:
Your fish produce ammonia. Ammonia builds up and can poison your fish causing them burns and maybe death. Cycling a tank means growing beneficial bacteria (BB) in your filter to process the ammonia. A set of bacteria grow and consume the ammonia and turn it into nitrite. Another set of BB grow and consume the nitrite and turn it into nitrate. Nitrate is then removed by weekly partial water changes (PWC
What happened when you changed filters is you threw out all your BB so now the ammonia is building up and poisoning your fish.