Thanks for the imput. It could have been poor filter maintenance or disturbing the soil. I plants that huge mat of HC, and I did not clean out the filter for 2 months. But I though I might have been ok because I only had fish in it for a few weeks at the time of the shrimp genocide. Is there one you think might be the main cuplrate? You don't think upping the dose was to blame. I am just trying to get a better understanding. Is a monthly filter cleaning called for, with new medium and stuff? I planed on doing a filter cleaning every other month.