fish death

Hi all, I'm sorry for the bold title but I'm in a bit of an emergency with my fish. The filter of my family's tank broke and my parents decided to install a new filter, as well as replace the majority of water in the tank to clean it (yes, I know, bad idea, but they didn't know). In reaction to...

I have had my tank established for at least four months now. I'm on my second filter cartridge but the bio wheel stayed put and I believe my tank to still be fully cycled. Ammonia levels are 0, Nitrites are 0, nitrates are 5 ppm, if that. It took about two months to get my tank fully cycled and...