have you considered that you may be over feeding your fish?
You mentioned that you have noticeable fish droppings in your tank. 2 things I would strongly consider if I was you is cutting baack on feeding. And the size of your cleanup crew. If the leftover food and detritus are simply accumulating in your tank, they will of course become suspended in the water.
I feed my fish once per week, and have never lost a fish to anything beside my leaving the strainer off of my intakes ! See my signature below.