Why can you not post pic's of this system??????
We are trying to diagnose a problem we've never seen. There's a reason why doctors will not treat you over the phone. We don't know anything about your system. We don't know what the sump looks like, how the filtration is set up, how the display is set up, what growths you have in the display........ Nothing. None of us can diagnose your problem, with any degree of certainty, without seeing the system.
Based on what you've said in this thread, I really don't see much of a mystery here. Nutrients build up, and animals die. You remove the dead or dieing animals, clean the system, and do some good sized water changes. The system runs fine for a while until nutrients build up again. Then more animals die, you clean up the system, and the whole precess starts all over again.