so I have this problem- I have a 30 gallon tank. I used to have a hang-on-the-side filter, things were good for years, the filter broke, so I replaced it with a fluval 205 canister filter; I dumped the ceramic bio media from the old filter into it, and it has sponges and a bag of mixed charcoal/ nitrogen removing media.
All is well, the transition to the new filter goes seamlessly. The instructions that come with the filter are a big vague on how often to replace media/ etc. so I decide to continue with my old schedule of once a week a 5% water change and once a month rinse the filter sponges and replace the charcoal, never touch the ceramic bio media.
After the first filter cleaning the tank crashed big-time. All the angel fish died within a day, the water was cloudy; I added bottled "bacteria for filters" and waited, and things got better. Did another filter maintenance, another crash. Decided to maintain filter once every three months. Have done that twice, and after each the tank has crashed. Last one was this friday; the only fish left alive are the cory catfish and an apparently indestrucible betta.
The fish go into distress almost immediately after my re-starting the filter, almost as if there is some horrible toxin being released? the school of tetras I put in to replace the angels were all dead within 6 hours after this last filter rinse.
Testing indicates the water is perfect, slightly hard perhaps 120 ppm
what am I doing wrong?