I only have this problem when cleaning the filter. The filter could sit for months without a fish lose. Since I got rid of the filter I'm running a Fluval FX6 and a Fluval 407.Your filter is not strong enough and cannot keep up with the bio load. What do you do with your filter media while you rinse the pads?
I just rinse the Phosphate pads in Tank water and the rest of the filter just sits out for a few minutes.