generally speaking how long are the fish (that died) kept in the bag before you add them to your aquarium? what type of inverts do you have? have you tested the ammonia level in your tank? the reason I ask, the "sump" you have seems more like an ammonia factory- can you test for this? critters have a weird way of adapting to their environments- some do well, others not so well. test for ammonia- as it is usually the culprit.