I've had my tank running for 3 months now and have 4 harlequins. The tank had cycled before I put my fish in. My nitrites and ammonia went up to 0.25. I did everything the fish slop told me too. Did water changes every 2 weeks so the good bacteria had time to build-up as they thought I was keeping my tank too clean. I also tried reducing feeding them, every other day. I was not over feeding them as the fish shop told me exactly how much to feed them. Nothing was working - the nitrites and ammonia wasn't going down, but they weren't going up either. All 4 fish stayed alive through the 3months. The fish shop said the last thing to try is a new filter in case mine just wasn't working properly. The 10gallon tank came with a interpet pf1 filter. I bought a interpet pf2 filter as that is the next size filter up and also has a separate biological chamber. I kept the old filter in the tank and just added the new filter as I planned to run both filters for 6weeks then gradually reduce the media in the old filter over 2weeks (after the 6weeks) till I was left with no media in the old filler. 12 hours after putting my new filter in 1 of my fish just dies. Could my new filter have killed my fish and will it kill the rest of my fish?